The US State Department said it was “deeply concerned” Wednesday over reports that an American journalist was kidnapped by pro-Russian separatists in Ukraine.

“We are deeply concerned about the reports of a kidnapping of a US citizen journalist in Slavyansk, Ukraine, reportedly at the hands of pro-Russian separatists,” State Department spokeswoman Jen Psaki said in a statement.

“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.

“We call on Russia to use its influence with these groups to secure the immediate and safe release of all hostages in eastern Ukraine,” she said.

“We have also raised our concerns with Ukrainian officials as they work with local authorities to try to de-escalate the security situation in and around Slavyansk.”

Pro-Moscow insurgents in Slavyansk — an eastern Ukrainian town which has become a tense flashpoint in recent days — are holding two journalists, an American working for Vice News, Simon Ostrovsky, and a Ukrainian working for a pro-Kiev outlet, Irma Krat.

Slavyansk’s local rebel leader Vyatcheslav Ponomarev told reporters that the American “is not being detained, was not abducted, has not been arrested” and claimed he was “working” in one of the rebel-occupied buildings.

However, the Twitter feed of the normally prolific journalist has been inactive for a day.

In a statement posted to its website, Vice News wrote that it is “aware of the situation, and 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.”