An Iranian-American journalist who once worked for a U.S. government-funded broadcaster is believed to have been detained by Iran for months now, authorities said, further raising the stakes as Tehran threatens to retaliate over an Israeli attack on the country.

The imprisonment of Reza Valizadeh was confirmed to The Associated Press by the U.S. State Department as Iran marked the 45th anniversary of the takeover of the American Embassy and hostage crisis on Sunday. It also followed a threat by Iranian Supreme Leader Ayatollah Ali Khamenei a day earlier to provide "a crushing response" to Israel and the U.S. as long-range B-52 bombers reached the Middle East in an attempt to deter Tehran.