A senior US official says the Trump administration has approved providing heavier weapons to Syria's Kurds as they move closer to the key Islamic State stronghold of Raqqa.
The Pentagon has long desired to have Arab elements of the Syrian Democratic Force (SDF) compose the ideal assault force for the upcoming Raqqa operation over the Kurds.
The YPG is an ethnic Kurdish militia tied to the Kurdistan Workers' Party (PKK), which has led an on-and-off insurgency in southeastern Turkey for three decades.
The Pentagon said last night that it would continue to prioritise them.
Ankara says the Kurdish militia known as the YPG, which forms the backbone of the force, is an extension of the Kurdistan Worker's Party, or PKK, which has been waging a decades-old insurgency in Turkey and is considered a terrorist group by Turkey, the USA and other Western countries.
But she called the Syrian Democratic Forces "the only force on the ground that can successfully seize Raqqa in the near future", an indirect pushback on Turkish proposals that the U.S. abandon its support of the YPG and instead rely on other Turkish-backed rebels for the crucial mission.
Charles Lister, a senior fellow at the Middle East Institute, said it was unclear how Washington can allay Turkey's concerns.
Turkish Prime Minister Binali Yildirim has spoken out against a US decision to arm Syrian Kurdish fighters, saying it can not use one terrorist group to try and defeat another.
Dana W. White, the Pentagon's chief spokeswoman, said in a written statement that President Donald Trump authorized the arms Monday.
"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", Defense Secretary James Mattis told reporters traveling with him on a stop in Denmark.
"The SDF, partnered with enabling support from U.S. and coalition forces, are the only force on the ground that can successfully seize" Raqqa in the near future, White added.
The issue risks further stoking tensions between Ankara and Washington less than a week before President Recep Tayyip Erdogan heads to Washington to meet his USA counterpart Donald Trump in their first face-to-face encounter as heads of state.
-Turkish tension. He said the USA wants to work alongside the Turks to battle IS.
Turkish warplanes carried out strikes on YPG forces in Syria last month and also hit Kurdish positions in neighboring Iraq, which Ankara described as "terrorist havens".
Turkey, the USA and other Western allies view the PKK as a terrorist group. "We are going to further accelerate this fight", Mattis said.