The United States on Thursday urged Ukrainian authorities to heed the demands of thousands of pro-EU demonstrators, as protesters kept up a blockade of top government buildings and occupation of a central Kiev square.

US Assistant Secretary of State Victoria Nuland, speaking at a meeting of the Organisation for Security and Cooperation in Europe (OSCE) in Kiev, said Washington stood with the Ukrainians dreaming of a European future after two weeks of opposition protests in Kiev and western Ukraine.

President Viktor Yanukovych's rejection of a key political and free trade deal with the EU and a crackdown on the ensuing peaceful protests have plunged the ex-Soviet country into its worst political crisis in a decade.

"We urge the Ukrainian government to listen to the voices of its people who want to live in freedom," Nuland said.

"This is Ukraine's moment: to meet the aspirations of the people, or to disappoint them and risk descending into chaos and violence."

German Foreign Minister Guido Westerwelle lobbed a thinly veiled jab at Russia, which threatened Ukraine with retaliation if it signed the EU deal.

"The threats and the use of economic pressure which we have seen over the last year are simply unacceptable."

But Russia's Sergei Lavrov, who was also attending the OSCE meeting, denounced European "hysterics" over Ukraine's rejection of the pact.

Yanukovych is on a visit to China and is expected to return to Ukraine on Friday.

Earlier Thursday, the Ukrainian leader met with President Xi Jinping and oversaw the signing of several energy, infrastructure and other deals worth $8 billion in a boost for the country's ailing economy.

Westerwelle visited the protests on Kiev's Independence Square late Wednesday and said the "gates of Europe" were still open for Ukraine.

Protesters have taken control of the iconic square and hoisted the star-studded blue flag of the EU and the red and black banner of the wartime anti-Communist Ukrainian Insurgent Army (UPA) on the city hall.

The demonstrators are led by an opposition coalition of nationalist leader Oleg Tyagnybok, politician Arseniy Yatsenyuk and world boxing champion Vitali Klitschko who heads the UDAR (Punch) party. They have demanded the resignation of the government and snap presidential elections.

'Extremists are not partners'

Addressing the OSCE meeting, Ukrainian Prime Minister Mykola Azarov said the government was "ready for dialogue" with its opponents.

But he stressed that EU leaders should discuss the agreements with the authorities, not the opposition.

"Nazis, extremists and criminals cannot be partners in Euro integration," Azarov told Westerwelle.

US Secretary of State John Kerry, who cancelled his visit to Ukraine, made a highly symbolic visit to neighbouring Moldova, which last month initialled the so-called Association Agreement with Brussels to set the country on the path to EU integration.

The protests, known as the "Euromaidan" after the local name for Independence Square (Maidan), are the biggest demonstrations in Ukraine since the 2004 Orange Revolution.

The "orange" revolt forced the annulment of fraud-tainted presidential elections initially claimed by Yanukovych and led to a new poll claimed by a pro-Western reformer.

First Deputy Prime Minister Sergiy Arbuzov, 37, seen as a key member of the so-called "Family" of close allies around Yanukovych, said the government does not rule out discussing snap elections with the opposition.

"We have to hold negotiations," he said on television late Wednesday. "Then, when there are official proposals, (we have to) discuss them."

But many protesters were sceptical. "They are not going to yield power, that's just talk," said Valeriy Vlasenko, standing in the snow on the Maidan.

Jailed opposition leader Yulia Tymoshenko called on the West to impose sanctions against the president and his family.

"Targeted sanctions against him and his family are the only language he understands," the former prime minister, who has launched a hunger strike in solidarity with protesters, was quoted as saying by her lawyer.

Ukraine's former presidents Leonid Kravchuk, Leonid Kuchma and Viktor Yushchenko on Wednesday united in an unusual call of support for the protesters.

Yushchenko, who was elected as a result of the "Orange" revolt, and Kravchuk have always backed the opposition and are staunch supporters of closer ties with the EU bloc.

But the involvement of Kuchma, president from 1994-2005, may be crucial given his close ties to Ukraine's powerful oligarchs and continued political influence.