Turkey's deputy prime minister says the US decision to supply weapons to Kurdish fighters battling Islamic State militants in Syria is "unacceptable".
Arming the Popular Protection Units (YPG) would "not be beneficial", Nurettin Canikli told A Haber TV.
Ankara says the YPG is an extension of the Kurdistan Workers' Party (PKK), designated a terrorist group by the US.
The PKK insurgency to secure Kurdish autonomy in Turkey has killed thousands of people.
Washington says the YPG is essential to the operation to capture the IS stronghold of Raqqa.
The YPG leads the Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias that has driven IS militants from about 6,000 sq km (2,300 sq miles) of northern Syria over the past two years with the help of US-led coalition air strikes and military advisers.On Tuesday, Pentagon spokeswoman Dana White announced that President Donald Trump had given his authorisation to "equip Kurdish elements of the Syrian Democratic Forces as necessary to ensure a clear victory" over IS in Raqqa.
Ms White also stressed the Pentagon was "keenly aware" of Turkey's concerns.
"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."
"The US continues to prioritise our support for Arab elements of the SDF. Raqqa and all liberated territory should return to the governance of local Syrian Arabs."
The Pentagon has previously armed only Arab elements of the SDF.(Source: BBC)