The U.S. journalist released by pro-Russian separatists on Thursday said he was beaten and tied up while detained for four days.

Simon Ostrovsky, a reporter for Vice News, in a short post Friday, described his four-day detention after being captured by pro-Russian forces in the town of Slovyansk in eastern Ukraine.

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He said he was accused of working for the CIA, the FBI and "Right Sector, a Ukrainian ultra-nationalist group." He described the cellar of an occupied government building, where he was held along with a dozen others.

"On Monday night, I was pulled out of a car at a checkpoint, then blindfolded, beaten, and tied up with tape," he said. "After spending hours alone on the floor of a damp cell with my hands tied behind my back and a hat pulled over my eyes, I was led into a room where I was accused of working for the CIA, FBI, and Right Sector, the Ukrainian ultra-nationalist group."

He said he was struck with a truncheon, when he refused to hand over his computer password but said, "as it turns out, I had it pretty easy because I was let go."

"Everyone being illegally held in that damp cellar, or any of the other buildings controlled by the self-proclaimed 'Donetsk People’s Republic,' should be released or handed over to the police immediately," he said, describing journalists, a local official and other activists who had been captured.

Ostrovsky said he found out after being released he was being held as a bargaining chip for the pro-Russian group when dealing with the government in Kiev.

The State Department had condemned the kidnapping earlier this week and called on Russia to use its influence in the region to help secure his release. The department said the kidnapping and other "recent hostage takings" directly violated an agreement to de-escalate the situation there between the United States, Russia, Ukraine and the European Union.

The United States has also accused Russia of reneging on the deal brokered in Geneva by not calling for the separatists to cease their occupation of government buildings and hand over their weapons in exchange for amnesty.

After a call with European leaders Friday, President Obama said the United States is prepared to impose further sanctions for Russia's latest action, with coordination of other international leaders.