Russian investigative journalist Grigory Pasko, who was jailed for espionage for exposing radioactive pollution by the navy, said Tuesday he was hit on the head by unidentified attackers in Siberia.

Pasko told Echo of Moscow radio station that two burly men approached him outside his hotel in the city of Barnaul where he was holding a seminar and one "hit me very hard on the temple."

The attackers shouted at him: "Get out of our city -- we'll get you!" said Pasko, who is the director of Foundation 19/29 that supports investigative journalists.

The Russian Union of Journalists said in a statement it was "concerned by what happened to Grigory Pasko and will follow how the situation develops."

Pasko was convicted of treason in 2001 after filming the Russian navy dumping radioactive waste in the Sea of Japan while working for its in-house newspaper and sharing the footage with Japanese television.

He was sentenced to four years in prison for espionage and high treason and was released on parole in 2003.

Pasko said the attack in Siberia happened a day after a local news site reported his arrival, calling him a traitor and a spy and threatening to follow and film his every move while he was in Barnaul.

Pasko said he had reported the attack and regional police confirmed to RIA Novosti state news agency that they had received his statement.