The United States is continuing to seek information about the whereabouts of Robert Levinson, a missing former FBI agent believed to be in Iran, the State Department said on Monday.

"Over the weekend, we did send another message via the Swiss to the Iranian government asking for any answer as to what they know of his whereabouts," State Department spokesman Sean McCormack told reporters.

It is the third time since Washington began seeking information from Iran about Levinson on March 12.

Washington sent about two weeks ago a formal letter to Iran through Switzerland because it has no diplomatic relations with Tehran since 1980, when it cut ties after the Iranian revolution.

"We would have hoped that the formal request ... would have been sufficient to launch a more formal investigation," McCormack said.

Levinson went missing in Iran early in March. The United States was "relatively confident" the former FBI agent did not leave Iran, the spokesman said.

Source: Xinhua