A Brooklyn-based journalist covering the crisis in Ukraine has not been seen since early Tuesday, with pro-Russia insurgents in the country alleging that the man is being held as a suspected spy.
Simon Ostrovsky, who works for VICE News, was working in the eastern city of Slovyansk.
The fluent Russian-speaker, who also holds an Israeli passport, has been covering the crisis in Ukraine for weeks and was reporting about the groups of masked gunmen seizing government buildings in eastern Ukrainian cities one after another.
Stella Khorosheva, a spokeswoman for pro-Russia insurgents in Slovyansk, said that Ostrovsky was being held at a local branch of the Ukrainian security service that gunmen seized more than a week ago.
"He's with us. He's fine," Khorosheva told The Associated Press. She dismissed claims that they were keeping Ostrovsky hostage, saying the insurgents were not seeking to "exchange him for someone."
She said he is suspected of spying for the Right Sector, a nationalist group that has occupied two buildings in Kiev for months, "and other enemy organizations."
"(We) need to be careful because this is not the first time we're dealing with spies," Khorosheva said when asked why Ostrovsky was held captive.
A VICE spokesman said the outlet is "aware of the situation and is in contact with the United States State Department and other appropriate government authorities to secure the safety and security of our friend and colleague."
In Washington, State Department spokeswoman Jen Psaki said U.S. authorities are "deeply concerned" about Ostrovsky's detention, which she said violated the agreement between Russia, Ukraine, the European Union and the Unites States reached last week.
"We condemn any such actions, and all recent hostage-takings in eastern Ukraine, which directly violate commitments made in the Geneva joint statement," she 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."
Since November, Ukraine has been engulfed in its biggest political crisis since the 1991 fall of the Soviet Union. Months of anti-government protests in Kiev culminated in President Viktor Yanukovych fleeing to Russia in late February.
Ukraine's acting government has accused Russia of orchestrating the unrest in eastern Ukraine, which it fears Moscow could use as a pretext for an invasion. Last month, Russia annexed Crimea weeks after seizing control of the Black Sea peninsula.