US President Donald Trump has approved arms supplies to Kurdish YPG fighters to support an operation to retake the Syrian city of Raqqa from Islamic State, despite fierce opposition from NATO ally Turkey.
Ankara views the Kurdish YPG militia, fighting within a larger US-backed coalition, as the Syrian extension of the Kurdish PKK militant group, which has fought an insurgency in south-eastern Turkey since 1984.
The Pentagon stressed it saw arming Kurdish forces "as necessary to ensure a clear victory" in Raqqa, Islamic State's de facto capital in Syria and a hub for planning the group's attacks against the West.
"We are keenly aware of the security concerns of our coalition partner Turkey," Pentagon spokeswoman Dana White said in a statement on Tuesday.
"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."
There was no immediate reaction from Turkey, with president Tayyip Erdogan expected to meet Trump in Washington next week.
The US has long directly supplied arms to Arab components of the so-called Syrian Democratic Forces, which include YPG fighters.
Ms White said Washington would still prioritise supplying those Arab fighters within the SDF.
The decision to arm the Syrian Kurds will likely cast a shadow over Erdogan's US visit, policy experts said.
"There have been bad episodes in the relationship between the United States and Turkey, but this one is serious because it gets to the heart of Turkish security priorities," director of the Turkey project at the Center for Strategic and International Studies in Washington, Bulent Aliriza, said.
"You've now got a question mark over the US-Turkish security relationship that is pretty serious."
Ankara has long argued Washington should switch support for the planned assault on Raqqa from the Kurdish YPG militia to Syrian rebels Turkey has trained and led against IS for the past year.
Ankara believes the YPG's advances will fuel anti-Kurdish sentiment in predominantly Arab parts of Syria such as Raqqa and threaten Syria's territorial integrity.
