The United States criticized Russian security officials Thursday for their treatment of Leonid Razvozzhayev, an opposition activist.

Razvozzhayev says he was seeking asylum at a United Nations office in Kiev, Ukraine, on Friday when Russian authorities seized him and tortured him, eventually forcing him to sign a paper confessing that he had worked with Georgian nationals to plan riots in Russia.

He turned up in Moscow on Sunday. As he was being escorted to a police car, Russian TV crew filmed him yelling about being kidnapped and tortured.

Razvozzhayev is now being held in a Moscow prison. Russian officials say he had turned himself in.

On Thursday, a spokesman for the U.S. Embassy in Moscow expressed concern.

“There have been several authoritative reports on Leonid Razvozzhayev, including statements from the United Nations High Commissioner for Refugees and from human rights activists who visited him in prison,” he said, according to Russian news outlet Ria Novosti.

“We are deeply concerned with these reports and are following the developments closely. Today we expressed our concerns to the Russian government and have requested for the issue to be investigated thoroughly.”

Opposition activists in Russia have become increasingly vocal this year. Things reached a tipping point in March, when thousands rallied to protest Vladimir Putin's election to a controversial third term as president.

High-profile arrests in recent months have garnered international attention. In July, for instance, three members of the punk band Pussy Riot went on trial after being arrested for putting on a disruptive anti-Putin performance in a Moscow cathedral. Two of the band members were sentenced to serve two years in a brutal penal colony -- this month, they launched an appeal and lost.

In July, Russia's best-known human rights activist, Alexander Navalny, was charged by state investigators with embezzlement. Navalny could face a decade in jail; he maintains that the charges are ridiculous and politically motivated. The investigations are ongoing.

Kremlin officials maintain that they are simply enforcing existing laws.

On Thursday, Russia’s Foreign Ministry issued a statement calling U.S. criticism of Razvozzhayev’s treatment “hypocritical,” according to Ria Novosti.

“Attempts by the United States to accuse Russia of a violation of its obligations on the international convention against torture are not only unfounded, but also hypocritical,” said the statement, which went on to accuse the United States of condoning acts of torture carried out by American military members.

Putin spokesman Dmitry Peskov was far more restrained.

“This is hardly the kind of thing the Kremlin can and should comment on,” he said. “This is a matter for investigation agencies, prosecutors, lawyers and human rights workers.”