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By Pavel Polityuk and Alastair Macdonald KIEV (Reuters) - Russian Foreign Minister Sergei Lavrov accused the United States of being behind the political upheaval in Ukraine and said Moscow would respond if its interests came under attack. Lavrov's comments came a day after U.S. Vice President Joe Biden was in the Ukrainian capital with promises of support for the pro-Western government, and a warning to Russia not to interfere in Ukraine. The crisis in Ukraine, now in its fourth month, has dragged Russia's relations with the West to their lowest since the Cold War. In the east, pro-Russian armed separatists have seized about a dozen public buildings and are defying Kiev's authority.