NEW YORK - AN IRANIAN-AMERICAN scholar has been arrested by Iranian authorities for the second time in two years, the State Department and the man's family said on Friday.

Security forces arrested Kian Tajbakhsh late on Thursday, a family member told The Associated Press. The relative was in contact with Mr Tajbakhsh's wife, who witnessed the arrest in Teheran.

State Department spokesman P.J. Crowley told reporters that the United States was aware of the arrest and was checking into the circumstances. He provided no further detail.

There was no comment from Iranian authorities. The relative spoke to the AP on condition of anonymity because of concern that public comments could aggravate Mr Tajbakhsh's situation. The arrest comes amid high political tension in Iran. Teheran has seen massive demonstrations following disputed June 12 presidential elections.

Opposition protests erupted again on Thursday in the capital, with marchers chanting 'death to the dictator.' In some places, clashes erupted as police fired tear gas and charged demonstrators with batons.

Mr Tajbakhsh, a social scientist and urban planner, spent four months in prison in 2007 on charges of endangering national security. He denied the allegations.

In the latest incident, two people who identified themselves as Iranian security officials arrived at his residence in Teheran late on Thursday, Mt Tajbakhsh's family said. The officials questioned him and his wife and searched the residence for three hours, before taking him away along with two computers and other items.

The family said that they have no information about where Mt Tajbakhsh was taken.

Mt Tajbakhsh was held by authorities in 2007 after being charged along with three other Iranian-Americans with endangering national security. Then, he was a consultant with the Soros Foundation's Open Society Institute, an organisation that Iran has accused of trying to undermine the Iranian government.

Mr Tajbakhsh's family says that he ended his association with the Open Society Institute after his earlier imprisonment.

Iran's arrest of Iranian-American journalist Roxana Saberi in January on spying charges sparked tensions at a time that US President Barack Obama was looking for openings to negotiate on Iran's nuclear ambitions. Ms Saberi was released in May ahead of the elections and returned to the United States. -- AP