An American journalist operating in eastern Ukraine has been kidnapped by pro-Russian gunmen, the separatists said Wednesday.

Simon Ostrovsky, working for Vice News, was seized at gunpoint early Tuesday by masked men in the restive eastern Ukrainian city of Slovyansk.

Stella Khorosheva, a spokeswoman for the insurgents, confirmed Wednesday that Ostrovsky was being held at the local branch of the Ukrainian security service, seized more than a week ago, according to The Associated Press.

"He's with us. He's fine," Khorosheva told the AP, who said the journalist was being held because he's "suspected of bad activities," which she refused to explain. She said insurgents were holding the journalist pending their own investigation.

State Department spokeswoman Jen Psaki said the U.S. is "deeply concerned" about the reports.

"We condemn any such actions, and all recent hostage-takings in eastern Ukraine, which directly violate commitments made in the Geneva joint statement," Psaki said.

The reports come in the same week as a visit by Vice President Biden to Ukraine.

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