U.S. has received indications recently that a former FBI agent who disappeared when visiting Iran four years ago is still alive, said U.S. State Secretary Hillary Clinton on Thursday.

Clinton said in a statement that the U.S. citizen, named Bob Levinson, is now being held "somewhere in southeast Asia."

"We respectfully request the Iranian government to undertake humanitarian efforts to safely return and reunite Bob with his family," she said, adding that the U.S. would appreciate the Iranian government's efforts in this regard.

Bob Levinson, an American private detective and former FBI agent, disappeared in March 2007 when he was visiting Iran.

Washington has repeatedly asked Tehran to make efforts to determine the whereabouts of Robert Levinson and ensure his rapid and secure return to the United States. But the Iranian authorities had denied knowledge of the case.