DETROIT — An interim nuclear agreement between world powers and Iran presents “the perfect time” to press for the release of an ex-Marine being held on spying charges, a former defense secretary and three retired high-ranking generals wrote to President Obama.

The letter, dated Monday and signed by one-time Secretary of Defense William Cohen and retired Gens. Peter Pace, James Jones and Joseph Ralston, calls on the White House to take “immediate action to facilitate the release” of Amir Hekmati, of Michigan, who has spent more than two years in prison in Iran.