Pro-Russian separatists from Ukraine have confirmed that they are holding American-Israeli journalist Simon Ostrovsky hostage.

Vyacheslav Ponomaryov, the separatist head of the eastern Ukrainian town of Sloviansk, told reporters Wednesday evening that Ostrovsky had been detained since Tuesday for reporting false information, Reuters reported.

Ponomaryov has appointed himself “mayor” of the town. Ostrovsky has been covering the conflict in Ukraine in recent months for the Vice.com news.

The news website said in a statement that: “VICE News 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.”

Separatist gunmen detained Ostrovsky on Monday night along with four other reporters who have since been released. He reportedly is one of 16 people kidnapped in the town over the last two weeks. Some were released and others were found dead, according to the Kyiv Post.

Without mentioning his name, State Department Spokeswoman Jen Psaki on Wednesday evening expressed on behalf of the department “deep concern about the kidnapping of a US citizen journalist in Slovyansk, Ukraine, reportedly at the hands of pro-Russian separatists. We have also raised our concerns with Ukrainian officials as they work with local authorities to try to de-escalate the security situation. We likewise condemn the taking of any hostages, including journalists in eastern Ukraine. We call for their immediate release and call on Russia to use its influence to ensure they’re freed immediately.”

Ukraine has seen violent clashes between pro-Russian protesters and other groups since the ousting in February of the government of former president Viktor Yanukovych in a revolution which erupted over his perceived pro-Russian policies. Ukraine’s interim government has announced new elections scheduled for next month.