Turkey has urged the US to stop backing the Kurdish YPG (People's Protection Units) in Syria, as it steps up an offensive against the militia.
A spokesman for President Recep Tayyip Erdogan said YPG fighters were using US-supplied weapons against troops trying to clear the Afrin region.
Turkey says the YPG is an extension of the Kurdistan Workers' Party (PKK), which is fighting for Kurdish autonomy in Turkey, and is a terrorist group.
The YPG denies any direct organisational links with the PKK.
The militia, which controls much of north-eastern Syria, has been a key US ally in the fight against Islamic State (IS) fighters there.
The UN Security Council discussed Turkey's growing offensive in a meeting on Monday, but did not condemn it.
Ankara is now demanding an end to the US alliance with the YPG, arguing that the fight against IS is over.
"We cannot tolerate the PKK establishing some kind of a state structure along our border in Syria," warned presidential spokesman Ibrahim Kalin.
Thousands of civilians are reportedly trying to flee Afrin, and Syrian activists say more than 70 people have died since the Turkish push began on Saturday.