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

The United States on Thursday urged Ukrainian authorities to heed the demands of thousands of pro-EU opposition 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 the US stood with the Ukrainians dreaming of a European future after two weeks of protests in Kiev.

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."

Several other influential foreign ministers, including Carl Bildt of Sweden, Radoslav Sikorski of Poland and Russia's Sergei Lavrov were also in attendance.

Yanukovych is now on a three-day visit to China where he hopes to win a new lifeline for the country's ailing economy.

Westerwelle visited the protests on Independence Square late on 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 to oust Yanukovych.