The United States is calling on Iran to release an Iranian-American scholar arrested last week in Tehran.

State Department spokesman Ian Kelly on Monday called the detention of Kian Tajbakhsh unjust and said the United States was "deeply concerned."

He added that it is "unfortunate that the Iranian government is making choices that only serve to isolate Iran from the international community."

Kelly also urged Iran to return the U.S. passports of Americans being kept in Iran on "groundless charges," including Iranian-American researcher Esha Momeni.

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