The European Union said it will suspend efforts to work with Ukraine on a trade and cooperation agreement, as 200,000 pro-EU protesters gathered for a rally, marking the start of a fourth consecutive week of anti-government unrest.
The EU said on Sunday that Kiev's government has "no grounds in reality".
Stefan Fuele, EU enlargement chief, said on Twitter that he told Ukraine's first deputy Prime Minister Serhiy Arbuzov that the bloc would discuss a trade agreement, if Ukraine's government showed a commitment to sign it.
But Fuele said he has not heard back, so negotiations remain on hold.
US senators John McCain and Christopher Murphy met with Ukraine's President Viktor Yanukovich on Sunday - after addressing the pro-EU crowds in Independence Square.
"The way towards change goes through peaceful protest," Murphy told the cheering crowds. "And in America we stand in awe of the fact that, despite the violence on the square you have remained peaceful in support of change."
McCain, a veteran of rabble-rousing US politics, told protesters that they were right to look to the west.
"I am a Republican, Senator Murphy is a Democrat - we are here together speaking for the American people in solidarity with you," he said.
"And the destiny you seek lies in Europe. Ukraine will make Europe better, and Europe will make Ukraine better... 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. A future in Europe. A future of peace. The free world is with you. America is with you. I am with you."
Meawhile, tensions are expected to mount yet further, as authorities organised a mass demonstration to counter the opposition's rally.
They too vowed to hold a "non-stop protest" and said they would bus thousands of Yanukovich supporters from the provinces to a park near the opposition camp.
Protesters have remained in Independence Square in the capital Kiev since Yanukovich turned down the EU trade deal in favour of fostering closer ties with Russia.
Both US and European politicians have shown their support for the opposition's cause.
McCain flew to Kiev on Saturday for meetings with former boxing champion Vitali Klitschko, head of the Ukrainian Democratic Alliance for Reform party (UDAR), nationalist leader Oleg Tyagnybok and Arseniy Yatsenyuk, the head of Yulia Tymoshenko's party.
He praised the protests as "an incredible show of patriotism".
President Yanukovich is under pressure to choose to align Ukraine with the West or with a Russian-led Customs Union.
He will travel to Moscow next week to meet with Russian President Vladimir Putin.
Ukrainian Prime Minister Mykola Azarov said on Sunday he hopes for a cheaper price for deliveries of Russian gas to Ukraine to emerge from the meeting, Interfax news agency reported.
"The price for Ukraine can be set at the level of average European prices," Azarov told TV broadcaster Inter in a late evening interview, according to Interfax-Ukraine.
This normally means a price of around $370-380 per 1,000 cubic metres - less than the $400 which Kiev pays now.