Razvozzhayev, a supporter of the opposition Left Front movement, said he was abused by Russian authorities after getting arrested in Ukraine on charges of supporting the ouster of Russian President Vladimir Putin.
Victoria Nuland, a spokeswoman for the U.S. State Department, said Washington had serious concerns about the treatment of opposition activists in Russia.
"We are quite concerned about allegations that he was forced to confess, that he may have been subjected to torture and we take concerns about this and other arrest actions taken against the May 6th protesters very seriously," she said.
Washington, she said, has shared its concerns with the Kremlin.
Putin's election to a third non-consecutive term in May sparked widespread protests in Russia. Razvozzhayev and Left Front leader Sergei Udaltsov were connected to an alleged coup plot alongside a politician in Georgia.
Vladimir Markin, a spokesman for a Russian investigative committee, was quoted in state media as saying Razvozzhayev was in his "right mind" when he signed a confession.
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