WASHINGTON (Reuters) - Iran has detained a fourth Iranian American, California businessman Ali Shakeri, the U.S. State Department said on Thursday, calling his arrest a "disturbing pattern" of harassment of dual citizens.

"He has also been taken into custody by Iranian officials," State Department spokesman Tom Casey told reporters, saying he believed Shakeri had been detained about 10 days ago and was being held in Tehran's Evin prison.

Shakeri is the fourth Iranian American to be detained, imprisoned or otherwise barred from leaving Iran in recent months in a series of incidents that U.S. diplomats fear may herald a further deterioration in U.S.-Iranian relations.

The other three -- academic Haleh Esfandiari, social scientist Kian Tajbakhsh and journalist Parnaz Azima -- have been charged with spying, a charge which could carry the death sentence under Iran's Islamic sharia law.

The United States has denied the three are U.S. spies and demanded their release, saying that they are private citizens who went Iran to meet family and have ordinary professional contacts.

Tehran accuses Washington of using intellectuals and others inside the country to undermine the Islamic Republic through what it calls a "velvet revolution."

Last week the Center for Citizen Peacebuilding at the University of California at Irvine, an institute that seeks to promote peace and where Shakeri serves on an advisory board, said he was in Iran and had not been heard from since March.

"What we are seeing is a disturbing pattern on the part of the Iranians of efforts to harass these innocent people," Casey said, saying he was aware of media reports Shakeri had been, or was on the verge of being, charged but could not confirm them.

The United States, which cut diplomatic ties with Iran after the 1979 Islamic revolution and the taking of hostages at the U.S. embassy there, is embroiled in disputes with Tehran over its nuclear program and its suspected support for the insurgency in Iraq.

Iran says its nuclear program is not to produce atomic bombs, as the United State suspects, but rather to generate electricity to it can export more oil and gas. It has also denied U.S. accusations it is fueling the violence in Iraq.

Despite the disagreements, the United States and Iran held their most high-profile talks in three decades in Baghdad on Monday. The talks focused on Iraq's sectarian violence and did not touch on issues like Tehran's nuclear ambitions.