Iran has released an Iranian-American academic who was working with a democracy advocate institute after he spent four months in a notorious prison on suspicion of trying to stir up a revolution, the official news agency reported Thursday.

Kian Tajbakhsh, an urban planning consultant with the Soros Foundation's Open Society Institute, was one of four Iranian-Americans charged with endangering national security — an accusation they and their employers denied.

The charges have increased tensions between the United States and Iran, already high over U.S. accusations that Iran was seeking to develop a nuclear weapons and is fueling violence in Iraq. Iran denies both claims.

But Iran appears to be trying to defuse the crisis. Tajbakhsh was the second of two U.S. citizens released from prison since August. Earlier this week, Iran also allowed a third Iranian-American — a journalist — to leave the country after authorities confiscated her passport in January.

"Kian Tajbakhsh was released from prison on bail," the official agency, IRNA, quoted the office of the spokesman of Iran's judiciary as saying late Wednesday. His bail was set at about $110,000, but he would not be allowed to leave Iran unless a judge grants him departure permission, IRNA reported.

Earlier this month, journalists were allowed to see Tajbakhsh during a visit to the notorious Evin prison. At the time, Tajbakhsh said he expected to be freed soon, and an Iranian judiciary spokesman had said his release was likely.

A telephone message left early Thursday with the New York-based Open Society Institute was not immediately returned. But earlier this week, the institute called for the immediate release of Tajbakhsh, saying he was held in solitary confinement at the prison.

In August, Haleh Esfandiari, director of the Middle East Program for the Washington-based Woodrow Wilson International Center for Scholars, also was released from Evin prison, where she had been held for months. Esfandiari left the country earlier this month and returned to the United States.

On Tuesday, Parnaz Azima, an Iranian-American, who worked for the U.S.-funded Radio Free Europe-Radio Liberty's Farsi-language service Radio Farda, was allowed to leave Iran. Authorities never imprisoned Azima but did not allow her to leave the country after confiscating her passport.

Still being held at Evin prison is Ali Shakeri, a member of a California-based democracy group, the Center for Citizen Peacebuilding. Iranian officials have not said anything about his release.