The ex-Soviet nation of 46 million has been at the heart of a furious diplomatic tug of war since President Viktor Yanukovych's shock decision last month to ditch a landmark EU association agreement and seek closer ties with old master Russia.
EU Enlargement Commissioner Stefan Fuele announced in a surprise tweet that the 28-nation bloc was halting all negotiations until it received "a clear commitment" from Yanukovych that Ukraine was serious about the deal.
Yanukovych is due in Moscow on Tuesday for talks with Russian President Vladimir Putin that protesters occupying central Kiev's iconic Independence Square fear could result in an even firmer alliance between the two neighbours.
Demonstrators have planned another huge rally to coincide with Yanukovych's meeting with Putin on Tuesday evening. They were warmly reassured of continued US backing today by Republican Senator John McCain -- one of Washington's staunchest critics of Kremlin rule.
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"To all Ukraine, America stands with you," McCain called out to a cheering sea of people who chanted "Thank you!" in English in return.
EU officials told Ukraine that further discussions required a "clear commitment (to) sign (but) Work on hold, had no answer," said Fuele.
A Ukrainian government spokesman quickly responded that Kiev remained serious about the negotiations and did not treat Fuele's tweet as the formal position of the bloc.
Prime Minister Mykola Azarov has requested a USD 27.5-billion loan from the European Union before it signs the closer trade and political association deal.
EU officials have rejected the loan request and argued that Ukraine stood to benefit over the long term from the removal of EU trade barriers.