WASHINGTON — The United States said yesterday (Aug 16) that it would withdraw two Patriot missile-defence 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 anti-missile systems would be needed elsewhere to defend against threats from Iran and North Korea.

A statement issued by the US Embassy in Ankara from the American and Turkish governments said the Patriots would be sent back to the United States for "critical modernisation upgrades". If needed in a crisis, the batteries and their 250 troops could be rushed back to Turkey "within one week" to fulfil an American and NATO commitment to Turkey's air defences. Air defences aboard US 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 US diplomats and other senior officials were so concerned that they delayed notifying Turkey of the impending Patriot pullout until after President Barack Obama and President Recep Tayyip Erdogan of Turkey reached agreement on the bases and other anti-terrorism steps in a phone call on July 22, senior US officials said yesterday.

"Before there was a deal, any announcement that could be perceived as the US diminishing its commitment to Turkey would have sent the wrong signal," said Mr Derek Chollet, a former assistant secretary of defence who is now a senior adviser for security and defence policy at the German Marshall Fund.

The challenge for senior State and Defense Department officials boiled down to managing one priority - withdrawing the anti-missile systems and their troops, an Army capability already stretched by near-constant deployments - without scuttling another priority, gaining Turkish permission to launch air strikes from Incirlik. Last week, six F-16 fighter jets and 300 US military personnel arrived at Incirlik and flew their first combat missions.

Internationally, the decision by Turkey and the United States to work together was hailed as a critical rapprochement between allies who had long disagreed on an approach to the civil war in Syria. The new mission is to clear the Islamic State from a 59.3km-long area along the Turkish border in Aleppo province and from territory about 48.2km south. But now, two weeks after the plan was announced, it is unclear which forces will fight the militants on the ground, what support they will receive and how long it will take, Turkish officials and Syrian rebel leaders say. Four US officials, speaking on condition of anonymity to discuss a diplomatic issue, said yesterday that Turkish officials were livid when told two weeks ago that the United States was withdrawing the Patriots. Patriot systems have been stationed just north of the Syrian border since January 2013.

The deployment was made at Turkey's request after the shooting down of a Turkish jet by Syrian forces in June 2012 and the killing of Turkish civilians by Syrian shellfire four months later as the civil war there escalated.

Turkey, which was supporting the Syrian opposition to President Bashar Assad, expressed concern at the time that it was vulnerable to Syrian missiles, including Scuds that might be tipped with chemical weapons.

Scud attacks by forces loyal to Mr Assad against rebels in northern Syria in late 2012 added to Turkey's concerns. The Scuds fired at the rebels were armed with conventional warheads, but the attacks showed that the Assad government was prepared to use missiles as it struggled to slow rebel gains.

Originally, three NATO allies - the United States, the Netherlands and Germany - each agreed to provide two Patriot missile batteries and about 250 soldiers to augment Turkey's air defences. The Dutch mission was replaced this year by a single Spanish battery.

The Defence Department never liked the Patriot mission, which many Pentagon officials viewed as mostly a symbolic gesture. The Patriots in Turkey have not been called on to intercept a single hostile missile. The batteries and their troops are among the most highly demanded in the Army, and as the missile threat from the Assad forces seemed to ease, Pentagon officials sought to withdraw the units, give their troops more rest at home and focus on higher-priority threats from Iran and North Korea.

"The Patriots are really stretched," said one senior US general familiar with the batteries' operations.

Several months ago, after an internal review of what the embassy statement yesterday called the "global missile defence posture," the administration decided to withdraw the Patriots from Turkey, Pentagon officials said. On Saturday, Germany said it would pull its batteries and troops from southern Turkey by the start of 2016 after a reassessment of the threats stemming from the conflict in Syria. "The decision was taken after the present assessments made by the NATO in June 2015, which concluded that the threat against Turkish territories by Syrian ballistic missiles is very low," Ms Ursula von der Leyen, Germany's defence minister, said in a statement.

Since the Turks were notified of the United States' decision, US officials have argued that the armed drones and F-16's deploying to Incirlik will provide a better overall security guarantee than the Patriot batteries.

But the strikes from Incirlik are aimed only against the Islamic State, also called ISIS or ISIL, and in support of the so-called moderate Syrian opposition. Their mission does not extend to knocking out Mr Assad's missile forces as part of the pledge to protect Turkey.

Many of Mr Assad's troops have pulled back from the border areas with Turkey, but US officials could not say yesterday if the Syrian army's Scud launchers had also been pulled back or if the army had run out of missiles that could reach Turkey. THE NEW YORK TIMES