Ukrainian officials say foreign security agencies are responsible for the disappearance of a Russian opposition activist, who said he was kidnapped and forcibly taken to Moscow.

An Interior Ministry spokesman said Thursday it is clear that criminal elements were not involved in last week's disappearance of Leonid Razvozzhayev, an assistant deputy of the opposition Just Russia party.

Razvozzhayev told human rights activists visiting him in detention earlier this week that he was abducted by masked men in Kyiv, where he had been seeking asylum. He said he was taken across the Russian border and held in a building for several days, where he was tortured into confessing to plotting mass riots against Russian President Vladimir Putin.

The activist's lawyer on Thursday said his client had retracted his confession, saying it had been made under pressure.

Russia's Investigations Committee said Razvozzhayev turned himself in on Sunday in Ukraine and admitted to involvement in organizing mass disturbances in Russia.

In Moscow, the U.S. embassy expressed concern to the Russian government about the activist's situation and requested an investigation into the case.

The United Nations refugee agency also voiced its concern, and stressed that as an asylum-seeker, Razvozzhayev is protected by international refugee law. The agency's Kyiv office on Monday confirmed that the activist disappeared after registering for asylum last week.

Razvozzhayev was featured in a pro-Kremlin documentary in which he and other activists appeared to plan mass riots and a coup in an effort funded by Georgian politician Givi Targamadze.

He faces up to 10 years if convicted.

Razvozzhayev's arrest follows a criminal probe launched last week by Russian authorities against two other opposition activists on charges they organized riots in May in Moscow. Left Front party leader Sergei Udaltsov was released and ordered to stay in Moscow, but his aide, Konstantin Lebedev, is in police custody.