Russia on Thursday refused to launch a probe into claims of an activist who says he was abducted by security forces from neighbouring Ukraine and tortured into confessing to a plot against President Vladimir Putin.

Leonid Razvozzhayev, a 39-year-old aide to an opposition lawmaker, is currently under arrest and awaiting trial on charges that he incited mass unrest against Putin together with other leftist activists.

His case has caused an international outcry over his claim that last month masked men kidnapped him from Ukraine, where he was applying for UN refugee status, smuggled him into Russia and tortured him until he confessed. He later retracted his confession.

The Investigative Committee said on Thursday it had conducted an "exhaustive probe" into the activist's claims, adding that the facts could not be confirmed.

"In this connection the investigative bodies decided not to open a criminal case," it said in a statement.

"Apart from his report, Razvozzhayev himself was unable to provide investigators with any documented or material evidence of the crime committed against him. Razvozzhayev rejected an offer to have his testimony confirmed with the help of a polygraph."

Investigators also said they had questioned a number of witnesses who confirmed the activist's claims were "unreliable," adding he had voluntarily crossed the border back into Russia on October 19.

At the same time, the Investigative Committee added, Razvozzhayev confirmed that he had illegally crossed the border into Ukraine on October 16 by using his brother's passport.

The General Prosecutor's Office has tasked the investigators with looking into the incident, the statement added.

The activist's lawyer, Dmitry Agranovsky, said the refusal to look into his client's claims was not surprising.

"According to Razvozzhayev, employees of the Investigative Committee were also involved, and the abductors directly passed him over to an investigator from the Investigative Committee," Agranovsky told AFP.

He also said the activist had not refused to undergo the polygraph test. "Taking into consideration his mental state at that moment, we said we needed additional consultations," he said. "Besides, the polygraph is not 100 percent accurate."

On Wednesday, Russian investigators also charged Razvozzhayev with an armed robbery in Siberia dating back nearly 15 years.

Russian authorities launched the probe against Razvozzhayev, Left Front leader Sergei Udaltsov, and his assistant Konstantin Lebedev after a pro-Kremlin channel aired a smear documentary alleging they had plotted a coup with financial backing from a Georgian lawmaker.