President Donald Trump authorized the Defense Department to equip "Kurdish elements" of the U.S.-backed Syrian Democratic Forces (SDF) on Monday "to ensure a clear victory" over Daesh in Raqqah, Syria, Pentagon spokeswoman Dana White said.
"The SDF, partnered with enabling support from U.S. and coalition forces, are the only force on the ground that can successfully seize" Raqqah in the near future, she added.
The announcement over Turkey's objections comes one week before Turkish President Recep Tayyip Erdogan is scheduled to visit Washington and meet Trump.
U.S. support for the SDF has been a major strain on relations between Washington and Ankara as the YPG forms the backbone of SDF forces.
Turkey considers the PYD and its armed wing, the YPG, to be the Syrian offshoots of the PKK, a proscribed terrorist organization in the U.S., Turkey and the EU.
But Washington has adamantly resisted similarly designating the YPG, using it as a principal partner in the ground war in the fight against Daesh in northern Syria.
The Pentagon said the U.S. is "keenly aware" of Turkey's security concerns as it announced the policy shift.
"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," White said.
"The U.S. continues to prioritize our support for Arab elements of the SDF," she said, adding that Raqqah "and all liberated territory should return to the governance of local Syrian Arabs."
The Pentagon did not immediately specify what types of support the U.S. would begin to supply under the new authorities, but a Defense official told Anadolu Agency it would include small arms, machine guns, ammunition, armored vehicles, trucks and engineering equipment.
The official spoke on condition of anonymity because he was not authorized to discuss the matter publicly.
Trump has prioritized the defeat of Daesh and the liberation of its de facto Syrian capital, Raqqah, since he came to office in January.
In a separate meeting on Tuesday in the Danish capital of Copenhagen, Defense Secretary James Mattis said the U.S. and Turkey would work together as operations to take the city continue.
"Our intent is to work with the Turks, alongside one another, to take Raqqah down, and we're going to sort it out and we'll figure out how we're going to do it," Mattis said.
Commenting on the standoff over U.S. support for the YPG, Mattis said Turkey and the U.S. are NATO allies and have been in useful discussions.
"That's not to say we all walk into the room with exactly the same appreciation of the problem or the path forward," he added.