“We are keenly aware of the security concerns of our coalition partner Turkey,” White said in a statement. “We want to reassure the people and government of Turkey that the U.S. is committed to preventing additional security risks and protecting our NATO ally.”

The decision, which was first reported by NBC, is sure to enrage Turkey, which views the Kurdish People’s Protection Units (YPG), which make up the largest share of the SDF, as an existential threat. Ankara has repeatedly rebuked the United States for supporting the YPG, which has emerged as the Pentagon’s premier partner force against the Islamic State in Syria.

Ankara sees the YPG as an extension of the banned Kurdistan Workers’ Party, or PKK, which is considered a terrorist group by both Turkey and the United States.

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The Turkish position has created a dilemma for U.S. military officials, who see no viable alternative force in Syria capable of and willing to mount an assault on Raqqa, a city where they say militants are plotting attacks against the West. Already, the YPG has received air support from the United States and, indirectly through Arab fighters, some U.S. weaponry.

Trump is expected to officially inform Turkish President Recep Tayyip Erdogan of his decision next Tuesday, when Erdogan visits the White House.

The Trump administration — nor the Barack Obama administration before it — had not made any secret of its intention to give the Syrian Kurds a primary role in attempting to seize Raqqa. Defense officials have said repeatedly the Raqqa operation would require direct weapons shipments and upgraded equipment as local forces maneuver though minefields and other obstacles leading into the city.

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While Turkish officials have continued to complain publicly about a strategy they say enlists one terrorist group to fight another, they have privately acknowledged that the matter appeared to be decided.

Even so, Turkey has continued to lobby the Trump administration to change course ahead of Erdogan’s visit, dispatching to Washington top Turkish officials, including Gen. Hulusi Akar, the military chief of staff, and Hakan Fidan, the intelligence chief. A Turkish delegation briefly met with Trump on Monday, according to a report in the Turkish Daily Sabah newspaper.

To soften the blow, senior U.S. officials have been in near constant contact with their Turkish counterparts to assure them the Kurdish troops will not have any role in stabilizing or ruling Raqqa after the operation. The force leading the battle into the city, officials have said, will comprise Arab fighters who also form part of the SDF.

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“The United States fully supports returning Raqqa to the care and governance of local Arabs,” White said. “We do not envision a long-term YPG presence, and governance in the city is acceptable or consistent with the wishes of the local population.”

Defense Secretary Jim Mattis spoke by phone with his Turkish counterpart, Fikri Isik, on Tuesday, White said, reiterating the U.S. pledge to seek to head off additional risks to Turkish security.

Such promises are unlikely to allay concerns in Turkey, which has charged that the YPG has used its battlefield gains to claim territory traditionally dominated by Syrian Arabs and Turkmen. Its goal, Erdogan has said, is to create a Kurdish canton that can join with PKK separatists in Turkey.

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Late last year, the Turkish leader moved military forces into northern Syria, ostensibly to fight against the Islamic State, but equally to ensure that YPG forces did not consolidate along Turkey’s southern border. More recently, Erdogan has suggested he would send Turkish troops deeper into Syria, toward Raqqa, despite American plans to support a Kurdish-dominated offensive.

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Speaking earlier Tuesday, Mattis suggested that the United States hoped to continue some sort of military partnership with Turkey in Syria.

“Our intent is to work with the Turks, alongside one another to take Raqqa down,” Mattis said during a news conference in Denmark. “We’re going to sort it out, we’ll figure out how to do it, but we’re all committed to it.”

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Mattis, who also met with Turkish Undersecretary of Defense Basat Ozturk on Tuesday, declined to elaborate on the possible Turkish involvement. “NATO allies stick together,” he said. “That’s not to say we all walk into the room with same appreciation of the problem.”

Officials said Trump’s decision authorizes Mattis to move ahead with arming the Kurdish fighters, rather than setting a specific timeline for doing so. A Defense official, speaking on condition of anonymity to discuss details of the new decision, said authorized weaponry would include small arms, ammunition, heavy machine guns, and equipment to counter vehicle-borne bombs, a tool frequently used by the Islamic State.

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In a reflection of the Trump administration’s attempt to minimize Turkish objections, White said equipment provided for the Raqqa operation would be “limited, mission-specific, and metered out incrementally as objectives are reached.”

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The White House decision comes as Turkey ramps up its military operations against PKK and YPG fighters in Iraq and Syria. Last month, Turkish warplanes launched assaults on Kurdish fighters in both countries, killing more than a dozen people, and prompting a public outcry from Washington. In the latest airstrikes, Turkey said that it had destroyed “PKK terrorist camps” in northern Iraq on Tuesday, according to Turkey’s semiofficial Anatolia news agency.

The attack on YPG partner forces was especially objectionable for American Special Operations forces, who are deployed alongside the Kurdish fighters in different areas of Syria. The SDF is now locked in a pitched battle with the Islamic State around the town of Tabqa on the Euphrates River, a battle that U.S. officials say is a key steppingstone to the Raqqa offensive.

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The U.S.-backed campaign against the Islamic State is just one of several parallel conflicts unfolding in Syria after more than six years of civil war.

Kareem Fahim contributed to this report from Istanbul. Gibbons-Neff reported from Vilnius.