The U.S. decided to pull its Patriot missile defense force out of Turkey mainly because it sees a declining Syrian military threat, including a depleted stockpile of Scud missiles capable of striking Turkish territory, the Pentagon said Monday.

The move is the latest twist in an evolving U.S.-Turkey partnership that is coping with the fallout from Syria's civil war. Just last month Turkey agreed to take a more active role in the U.S.-led campaign against the Islamic State group in Syria and Iraq and to allow the U.S. to use its bases for airstrikes. Last week the U.S. military announced that six F-16 fighter jets had arrived at Turkey's southern Incirlik air base, and they began flying missions over Syria on Wednesday.

U.S. officials have said they also are considering deploying combat search-and-rescue aircraft and personnel to Turkey, now that the F-16s are flying, and Turkey and the U.S. are in talks on how to integrate Turkish warplanes into the U.S.-led air campaign.

A Pentagon spokesman, Navy Capt. Jeff Davis, said Monday that a battery of Patriot missiles operated by the U.S. Army would be withdrawn from Turkey in October and not replaced. He said the battery would return to the U.S. for upgrades and that the move will not lessen the U.S. commitment to helping defend Turkey against threats from Syria.

Patriots, which are designed to defend against aircraft as well as ballistic missiles, were first sent to Turkey in January 2013 as part of a NATO response to Turkey's request for support against what it saw as a threat from Syrian missiles.

"The threat calculus has changed somewhat since 2013 as we we've seen the Assad regime grow weaker and as we've seen them shoot through the majority of their missile stock at targets there domestically," Davis told reporters.

The Patriots were never fired, he said.

Davis declined to provide specifics on the remaining Syrian missile stockpile but said it has diminished greatly since the early period of the Syrian civil war when the Bashar Assad government frequently fired Scud missiles at rebel forces.

"We've seen that drop off now," Davis said.

The spokesman said the changed circumstances in Turkey led to a judgment that the Patriots could be withdrawn this fall to upgrade them while Patriot missile batteries remain deployed in other parts of the world facing a bigger threat.

"For now, we don't necessarily assess this as being the highest priority globally for where these missiles should be," he said.

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AP Broadcast correspondent Sagar Meghani contributed to this report.