Simon Ostrovsky
Simon Ostrovsky@SimonOstrovsky

Pro-Russian rebels in the Ukrainian eastern city of Sloviansk have confirmed that they are holding American journalist Simon Ostrovsky captive.

Ostrovsky, a reporter for Vice, was being detained at the local branch of the Ukrainian security service, which was seized by pro-Russian insurgents, according to spokeswoman for the group Stella Khorosheva.

The journalist was taken hostage because "suspected of bad activities" according to Khorosheva. She refused to elaborate.

Ostrovsky was being held pending an investigation by the insurgents.

The US state department told BuzzFeed it was "aware" of the kidnapping.

Organisation for Security and Co-Operation in Europe representative on Freedom of the Media, Dunja Mijatovic, expressed "deep concern" about the case.

"I remain deeply concerned about the ongoing negative pattern in relation to journalists' safety in Ukraine," Mijatović said.

"I call on all those responsible to stop harassing and attacking journalists and let them do their job. Simon Ostrovsky should be released immediately."

The detention was announced by Vyacheslav Ponomarev, self-proclaimed "People's Mayor" of Sloviansk.

In a statement, Vice News says it "is in contact with the US State Department and other appropriate government authorities to secure the safety and security of our friend and colleague, Simon Ostrovsky".