Ukraine's SBU state security service said on Friday it was looking into claims that a Russian opposition activist was abducted in Kiev a week ago before being taken to a Moscow detention centre.
Activist Leonid Razvozzhayev disappeared last Friday after consulting with a partner organisation of the United Nations' refugee agency.
Rights workers who met Razvozzhayev say he has described being abducted, bound with tape and taken to a basement on his way to Moscow, where he was forced by masked men to confess to plotting unrest.
Russian investigators said Razvozzhayev had been charged with preparing mass disorder against the rule of President Vladimir Putin, a day after they said he had voluntarily signed a confession.
"A number of emotional statements, versions and theories have appeared in the media that often contradict each other," the SBU said in a statement.
"The SBU is carrying out checks with regards to reasons for Razvozzhayev's entry to Ukraine, the legality and (legal) status of his stay on our country's territory and the circumstances and the goal of his urgent departure to Russia."
Ukrainian police earlier said they had no formal reason to investigate the activist's disappearance as no kidnapping report had been filed.
The European Union has urged Ukraine to look into the case. "The allegations are serious and many questions need to be clarified," a spokesman for the EU delegation to Ukraine said.
Ukraine holds a parliamentary election on Sunday. Opposition parties are trying to wrest a parliamentary majority out of the hands of President Viktor Yanukovich's Party of the Regions.
"Foreign security services are operating in Ukraine," opposition leader Arseny Yatsenyuk said in a statement posted on his party's website this week.
"I guess they will also take part in counting votes cast by Ukrainians," he said. Yanukovich has sought a political rapprochement with Russia since becoming president in February 2010.
Olexander Yefremov, the leader of the Party of the Regions' parliamentary faction, told local media this week that Razvozzhayev's case was a "disservice" to Ukrainian authorities on Moscow's part.