The US will begin to directly arm the YPG, the Pentagon confirmed on Tuesday.
President Donald Trump authorised the defence department to equip "Kurdish elements" of the US-backed Syrian Democratic Forces (SDF), of which YPG forms the backbone, "to ensure a clear victory" over Daesh in Raqqa, Syria, Pentagon spokeswoman Dana White said.
The announcement came a week before Turkish President Recep Tayyip Erdogan is scheduled to visit Washington and meet with Trump.
US support for the YPG has been a major strain on relations between Washington and Ankara in the past few years.
TRT World's Kevin McAleese has more from Washington DC.
Turkey considers the PYD and its wing, the YPG, to be the Syrian offshoots of the PKK, a proscribed terrorist organisation by the US, the EU and Turkey.
But Washington has adamantly resisted similarly designating the YPG, supplying it with ammunitions and heavy weapons, and using it as a principal partner in the ground war in the fight against Daesh in northern Syria.
The Pentagon said the US 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 US is committed to preventing additional security risks and protecting our NATO ally," White said.
"The US continues to prioritise our support for Arab elements of the SDF," she said, adding that Raqqa "and all liberated territory should return to the governance of local Syrian Arabs."
"The SDF, partnered with enabling support from US and coalition forces, are the only force on the ground that can successfully seize" Raqqa in the near future, White added.
Turkey says that the Turkish-backed rebels are more effective in the fight against Daesh.
The Pentagon did not immediately specify what types of support the US would begin to supply under the new authorities, but a defence department official told Anadolu Agency it would include small arms, machine guns, ammunition, armoured vehicles, trucks and engineering equipment.
US to work together with Turkey
Separately, Defense Secretary James Mattis said the US 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 Raqqa down, and we're going to sort it out and we'll figure out how we're going to do it," Mattis said in the Danish capital Copenhagen.
Commenting on the standoff over US support for the YPG, Mattis said Turkey and the US are NATO allies and have been in useful discussions.