U.S. Sen. John McCain, right, shakes hands with a woman in Independence Square in Kiev, Ukraine, on Sunday, December 15, during a mass rally of the Ukrainian opposition. McCain told more than 200,000 EU supporters rallying in the Ukrainian capital that Washington backed their desire for European integration.
A Ukrainian pro-European protester holds a picture of jailed opposition leader Yulia Tymoshenko during a rally in Independence Square on December 15. Pro- and anti-government protesters massed in Kiev amid questions about the country's future ties to the West and Russia.
Ukrainian opposition supporters wave flags during a large rally in Independence Square on December 15.
Ukrainians sing the national anthem during a pro-European Union rally in Kiev on December 15.
Ukrainian opposition supporters gather at a mass rally in Kiev on December 15.
Ukrainian opposition supporters gather at a mass rally in Independence Square on December 15.
Supporters of the pro-European Union opposition cook on Saturday, December 14, in Independence Square in Kiev, Ukraine. They have occupied the square for three weeks, resisting police and refusing to leave.
Demonstrators by the thousands rally in Independence Square.
People go through a police line separating the competing rallies of the opposition protesters in Independence Square and supporters of Ukrainian President Viktor Yanukovych in European Square, in Kiev on December 14.
Supporters of the president wave flags of the ruling Party of Regions, as well as Ukrainian flags, during a rally on European Square in Kiev on December 14.
Anti-government protesters guard a barricade designed to keep police from evicting them from Independence Square on Friday, December 13, in Kiev, Ukraine.
Protesters stand at a barricade in Kiev on December 13.
Protesters hold Ukrainian national flags as they picket the Ukrainian House during round table talks between the Ukrainian President Viktor Yanukovych, former presidents and leaders of the opposition on December 13.
Riot police storm barricades set up by pro-European Union protesters in Independence Square in Kiev, on Wednesday, December 11.
Riot police move against protesters on Independence Square in Kiev around 2 a.m. December 11. U.S. Secretary of State John Kerry has expressed "disgust" over the crackdown.
A protester calls for attention inside the City Hall after riot police were forced out from blocking the front door on December 11.
Police enter Kiev's Independence Square on December 11.
Riot police officers block the entrance of City Hall as protesters pour water from windows and throw smoke grenades at them on December 11.
A young police officer guards a roadblock on Institutskaya Street near the presidential area in Kiev on December 11. Riot police tried to break into City Hall, which is held by protesters, but they were forced to retreat.
Protesters who oppose President Viktor Yanukovych, who is allied with Russia, clash with anti-riot police on Independence Square in Kiev on December 11.
A protester shouts behind riot police standing guard in Independence Square in Kiev on Monday, December 9.
A protester slams a toppled monument of Vladimir Lenin in Kiev on Sunday, December 8.
Riot police separate pro-EU protesters and supporters of the ruling Regions Party during a large rally held in Independence Square in Kiev on December 8.
Demonstrators shout slogans during the mass rally on December 8.
Thousands of protesters gather in Kiev's Independence Square on December 8.
Pro-European Union activists shout slogans during the rally on December 8. An estimated 100,000 Ukrainians participated.
A young protester shouts slogans near a placard depicting Russian President Vladimir Putin and signed "Fare you well!" during the December 8 rally.
Pro-European Union activists gather around a huge poster of jailed former Ukrainian Prime Minister Yulia Tymoshenko on December 8.
Riot police block protesters during the December 8 rally in Kiev.
A rose, the symbol of the revolution, lies on barricades being built by Pro-European Union activists next to the Ukrainian Government building in Kiev on December 8.
A pro-European Union activist holds a cut-out paper heart as she faces police at the presidential office in Kiev, Ukraine, on December 8.
European Union and Ukrainian national flags fly above a crowd of pro-EU activists in Independence Square on December 8.
Pro-EU activists shout slogans during the rally in Independence Square on December 8.
Demonstrators gather in Independence Square during the mass rally December 8.
A pro-EU activist offers flowers to police officers at the presidential office in Kiev on December 8.
Anti-government protesters camp in Independence Square early in the morning on December 8.
Protesters demonstrate against Yanukovich and call for a closer relationship to the EU on Independence Square in Kiev, Ukraine, on Saturday, December 7.
Protesters prepare food at a camp at central Independence Square on December 7, as thousands keep their vigil in Ukraine's capital.
Pro-EU activists attend a rally in Independence Square on December 7.
Orthodox believers, carrying icons and crosses, walk during a religious procession outside the parliament building in Kiev, Ukraine, on Friday, December 6.
Police stand guard opposite a sea of protesters near the Ukrainian parliament in Kiev on Tuesday, December 3. Riot police lined up to protect the office of President Viktor Yanukovich, whose decision not to sign a landmark trade deal with the European Union sparked the public outrage.
Protesters chant slogans outside the parliament in Kiev on December 3.
Protesters gather in front of the parliament in Kiev on December 3.
Police stand guard outside the parliament in Kiev on December 3.
Protesters use a bulldozer during clashes with police at the presidential office in Kiev, Ukraine, on Sunday, December 1. At least 100,000 anti-government protesters packed Independence Square on Sunday.
A protester throws stones toward riot police on December 1. The crowd chanted "Revolution!" and "Down with the Gang" as it gathered in Kiev's iconic Independence Square and steered a bulldozer within striking distance of police barricades protecting the nearby presidential administration office.
A bleeding protester shouts at a police medic after police pushed protesters off the street leading to the presidential administration building on December 1.
Protesters clash with police guarding the presidential administration building on December 1.
A Ukrainian protester throws stones at riot police during the clashes outside the president's office on December 1.
Police and protesters look at an injured person near the presidential office on December 1.
Pro-European Union demonstrators clash with police near the presidential administration office in Kiev on December 1.
Demonstrators gather in downtown Kiev on December 1.
Anti-government protesters gather near Shevchenko University on December 1 in Kiev.
A protester injured in clash with police stands on Independence Square in Kiev, Ukraine, on Saturday, November 30. Protesters gathered in the main square to protest the government's decision not to sign a landmark trade deal with the European Union.
Police officers guard Independence Square on November 30 after dispersing a rally.
Dozens of protesters were wounded in a clash with police in Kiev on November 30.
Ukrainian riot police officers detain a protester on November 30.
Opposition supporters hold flags of the European Union on November 30 as they guard the gates of the Mikhailovsky monastery.
A woman cries during a rally on November 30.
Injured protesters receive medical help in an ambulance after riot police broke up a rally on November 30.
Protesters gather over barrels with bonfires to warm themselves on November 30.
Opposition supporters shout slogans and wave flags on Friday, November 29.
A demonstrator holds a torn portrait of Ukrainian President Viktor Yanukovych on November 29.
Demonstrators hold hands during a rally on November 29.
Demonstrators take to the streets in the center of Kiev on Thursday, November The European Union has halted work on a trade agreement with Ukraine, an official said Sunday, after Kiev failed to show "clear commitment" to signing the deal.
Stefan Fule, European commissioner for enlargement and European neighborhood policy, said on Twitter that the words and deeds of Ukrainian President Viktor Yanukovych and his government on the proposed pact were "further & further apart. Their arguments have no grounds in reality."
Fule said he had told Ukrainian Deputy Prime Minister Serhiy Arbuzov in Brussels, Belgium, last week that further discussion on the agreement was conditional on a clear commitment by Kiev to sign the deal, but he had received no response.
"Work on hold, had no answer," he tweeted.
Last month, Kiev spurned the agreement in favor of closer economic ties with Moscow, sparking mass protests as thousands of people poured onto the streets of the capital, angered by the government's U-turn away from integration with Europe.
Setting up tents and barricades, they have stood their ground in Kiev's Independence Square, or Maidan, paralyzing the center of the capital.
On Sunday the protesters were addressed by U.S. Sen. John McCain, who has been critical of Ukrainian authorities' use of force against demonstrators.
"People of Ukraine, this is your moment. This is about you, no one else. This is about the future you want for your country. This is about the future you deserve," he said to loud cheers.
"A future in Europe, a future of peace, with all of your neighbors. The free world is with you, America is with you, I am with you."
The demonstrations are the biggest since the Orange Revolution overturned the results of the eastern European country's 2004 presidential election.
East vs. West
The EU had kept its offer on the table, and the bloc's top diplomat, Catherine Ashton, on Thursday said Yanukovych had assured her Kiev intended to sign the deal after she met with him.
A day earlier, Prime Minister Mykola Azarov told a government meeting Ukraine was still open to signing the European integration deal, if the European Union would agree to provide financial assistance to Ukraine of around 20 billion euros ($27.5 billion).
On Sunday however, an EU diplomat told CNN there was "lack of clear commitment and timetable".
"We have had no reply (on timetable) from the Ukrainian authorities and so there is no basis to continue work," the diplomat added.
Ukraine is split between pro-European regions in the west of the country and a more Russia-oriented east.
Protesters say an EU agreement would open borders to trade and set the stage for modernization and inclusion. They accuse Yanukovych of preparing to take the country into a Moscow-led customs union.
Moscow has leverage that may have affected Yanukovych's decision last month to backpedal on the EU talks because Russia supplies Ukraine with natural gas.
The EU is also pressuring Yanukovych to free his chief political opponent, Yulia Tymoshenko, who has languished in jail for two years after being convicted of abuse of power in 2011. The EU and other critics decried the verdict as a sham.
The Orange Revolution that swept Yanukovych from office in 2004 also brought the pro-Western Tymoshenko to power.
At the rallies in Independence Square, protesters have carried her picture.
Investigation
Kiev's handling of the protests has been met with stern responses from Western governments.
In an overnight crackdown last week, police tore down barricades the protesters had set up. Before that, violence had broken out at a previous demonstration.
Ukrainian prosecutors are investigating some top government officials over the beating of students at anti-government protests two weeks ago.
The subjects of the criminal investigation include Ukrainian Deputy Secretary for National Security Vladimir Sivkovich, Kiev Police Chief Valery Koryak and Kiev Mayor Oleksandr Popov. A fourth person also is under investigation.
The case will be put before a judge on Monday, who could order house arrest for those being investigated, said Valery Vilkova, spokeswoman for the general prosecutor.