SLAVIANSK, Ukraine (Reuters) - Pro-Russian separatists in eastern Ukraine on Wednesday confirmed they are detaining a U.S. journalist working with Vice News.

The self-declared separatist mayor of Slaviansk told reporters the journalist, Simon Ostrovsky, had been detained for reporting what he said was false information that was "destabilizing for us" but that he was being treated well.

Mayor Vyacheslav Ponomaryov attempted to joke about the situation.

"There's nothing wrong with Ostrovsky. He is with us, he is feeling well and in a clean place. He is not a hostage but our guest. We only gave him a place of residence."

Gunmen detained Ostrovsky on Monday night along with other reporters who have since been released.

Vice News has said on its website that it is in contact with the U.S. State Department and other government authorities to work toward securing the safety of its journalist.

State Department spokeswoman Jen Psaki expressed deep concern at the kidnapping of the U.S. journalist and condemned the taking of any hostages in eastern Ukraine.

"We call for their immediate release and call on Russia to use its influence to ensure they area freed immediately," Psaki said at a news briefing in Washington.

(Reporting by Aleksandar Vasovic in Slaviansk; Additional reporting by Doina Chiacu in Washington; Writing by Alissa de Carbonnel; Editing by Angus MacSwan and Chizu Nomiyama)