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.
On Wednesday morning, the Turkish government denounced the US decision.
“We cannot accept the presence of terrorist organisations that would threaten the future of the Turkish state,” Mr Canikli told A Haber in an interview.
“We hope the US administration will put a stop to this wrong and turn back from it. Such a policy will not be beneficial – you can’t be in the same sack as terrorist organisations.”
The YPG and its political arm, the Democratic Union Party (PYD), denies any direct links with the PKK, which is waging an insurgency in Turkey for Kurdish autonomy.
The BBC’s Mark Lowen in Istanbul says the US decision is a huge slap in the face of its ally, which has consistently said arming the Kurds would be its “red line”.
Ankara is particularly irked that it was announced days before President Recep Tayyip Erdogan visits Mr Trump in Washington and as Turkish high-level officials were there urging the White House not to go ahead, our correspondent adds.
Mr Erdogan has argued that the YPG should be excluded from the Raqqa offensive and urged the US to switch to supporting a Syrian Arab rebel force that has driven IS out of a border area in the past year with the help of the Turkish military.
Ms White said the equipment provided to the Kurds – who make up 40% of the SDF’s 50,000 fighters, according to US-led coalition officials – would be “limited, mission specific and metered out incrementally as objectives are reached”.
She did not mention what would be provided, but other US officials indicated that it might include 120mm mortars, machine-guns and lightly-armoured vehicles.
A senior Kurdish official told the Associated Press that the US decision carried “political meaning” and “legitimise the YPG and the Syrian Democratic Forces”. But Ilham Ahmed warned it would likely be met with “aggression” from Turkey.
Last month, Turkish air strikes in northern Syria and Iraq’s Sinjar region killed at least 20 members of the YPG and five Iraqi Peshmerga fighters.