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Meanwhile, a senior aide to President Vladimir Putin warned that Russia could intervene to ensure Ukraine's security.
Dmytro Bulatov emerged bloody and bruised in Kyiv last week after going missing for more than a week during the mass protests in Ukraine.
Now, the Ukrainian activist is receiving medical treatment in Lithuania and told reporters he lied to his captors about being an American spy.
"They nailed me to the wooden door, I was kneeling and after that they took sticks and started beating me," Bulatov told reporters in the Lithuanian capital of Vilnius.
"I told them that the American ambassador had given me 50,000 dollars," he said. "It was so scary, it was so painful that I asked them to kill me. I lied because I could not stand the pain."
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He believes his captors were Russian secret service agents, and he accused them of cutting off part of his ear.
"They forced me to speak in front of the camera and they made me say everything they wanted; that I am an American spy...that America gives money for the Maidan (protest) and that I was hired by Americans in order to create disorder against our government."
Lithuania, after conducting a thorough medical examination of Bulatov, said it suspected he had been tortured.
Meanwhile, an adviser to President Vladimir Putin warned that Russia could intervene to maintain security in Ukraine unless the anti-government protests die down.
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Sergei Glazyev, who is responsible for Russia's relations with Ukraine, responded to a question about whether Russia might "actively intervene" by bringing up the Budapest Memorandum of 1994: "Under the document, Russia and the USA are guarantors of the sovereignty and territorial integrity of Ukraine and ... are obliged to intervene when conflict situations of this nature arise."
"And what the Americans are getting up to now, unilaterally and crudely interfering in Ukraine's internal affairs, is a clear breach of that treaty," Glazyev said. "The agreement is for collective guarantees and collective action."
Putin is expected to meet Ukrainian President Viktor Yanukovych in Sochi, while the latter attends the Winter Olympic Games.
Agence France-Presse and Reuters contributed to this report.