WASHINGTON — The United States said Sunday that it would withdraw two Patriot missile-defense batteries from southern Turkey this fall, a sign that the Pentagon believes the risk of Syrian Army missile attacks has eased since the Patriots were deployed in 2013. Officials said the antimissile systems would be needed elsewhere to defend against threats from Iran and North Korea.

A statement issued by the United States Embassy in Ankara from the American and Turkish governments said the Patriots would be sent back to the United States for “critical modernization upgrades.” If needed in a crisis, the batteries and their 250 troops could be rushed back to Turkey “within one week” to fulfill an American and NATO commitment to Turkey’s air defenses. Air defenses aboard American warships in the region also could help carry out the security mission over Turkey, the statement said.

What the statement did not mention was a complicated story behind the Patriot decision, which briefly last month seemed poised to dash months of painstaking talks with Turkey about allowing American armed drones and fighter jets to fly combat missions against the Islamic State from Incirlik Air Base and other Turkish installations.

American diplomats and other senior officials were so concerned that they delayed notifying Turkey of the impending Patriot pullout until after President Obama and President Recep Tayyip Erdogan of Turkey reached agreement on the bases and other antiterrorism steps in a phone call on July 22, senior American officials said Sunday.