Turkey on Wednesday reacted furiously to a US plan to arm Syrian Kurdish fighters whom Ankara considers terrorists, but the force battling Islamist militants said the move would hasten the extremists' defeat.
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 US counterpart Donald Trump in their first face-to-face encounter as heads of state.
The Kurdish Peoples' Protection Units (YPG) is seen by Washington as the best ally against Islamic State (IS) militants in Syria and the prime attacking force in any assault on their stronghold of Raqa.
But Ankara regards the group as a terror group and the Syrian branch of the Kurdistan Workers' Party (PKK), which since 1984 has waged an insurgency inside Turkey leaving tens of thousands dead.
The dispute poisoned ties between Turkey and the United States under the administration of former president Barack Obama but commentators had hoped for a smoother ride under Trump.
"The supply of arms to the YPG is unacceptable," Deputy Prime Minister Nurettin Canikli told A Haber television.
"Such a policy will benefit nobody," he said. "We expect that this mistake is to be rectified."
In a surprise announcement, the Pentagon said Trump had authorised the arming of Kurdish fighters within the Syrian Democratic Forces (SDF) "to ensure a clear victory over ISIS in Raqa."
The SDF, a US-backed group dominated by YPG but which also contains Arab elements, hailed the move and said that receiving US arms and military equipment would "hasten the defeat" of the jihadists.
"The US decision to arm the YPG... is important and will hasten the defeat of terrorism," an SDF spokesman told AFP.
The Kurdish-Arab SDF has been the main faction fighting IS on the ground in Syria, but Turkey dismisses it as a front for the YPG.
It remains to be seen what shadow the issue will cast over the talks between Trump and Erdogan, which have been touted as chance to forge a new partnership between the two NATO allies.
A high-level Turkish delegation including Chief of Staff General Hulusi Akar, presidential spokesman Ibrahim Kalin and Turkey's spy chief Hakan Fidan has been in the US laying the groundwork for the meeting.
Canikli said Turkey would take the diplomatic initiative to convince Washington "to do what friendship requires."
"The argument that a ground operation in the fight against Daesh (IS) would be successful only with YPG has nothing to do with reality," he said.
Both Washington and Brussels classifies the PKK as a terror group but has never regarded the YPG as such.
Turkey has said it is keen to join the battle to recapture Raqa but on condition the offensive does not include Syrian Kurdish fighters.
Last month, Erdogan said if Turkey and the United States joined forces, they could turn Raqa into a "graveyard" for militants.
Turkey is hoping for better relations with Washington after tensions during the Obama administration but their disagreement over Syrian Kurdish forces has proven to be one of the thorniest issues.
On April 27, Turkish warplanes struck YPG forces in Syria and also hit Kurdish forces in neighbouring Iraq in what Ankara described as "terrorist havens".
Pentagon chief Jim Mattis had on Tuesday taken part in a summit in Copenhagen for senior leaders from the top 15 countries in the anti-IS coalition, including Turkey.
Mattis, who is now in Vilnius as part of a European tour, gave a positive assessment of the role Turkey will play in the lead up to the Raqa fight.
"Our intent is to work with the Turks, alongside one another to take Raqa down," he said.
Meanwhile, Trump was to receive Russia's top diplomat Sergei Lavrov at the White House on Wednesday to discuss a Russian plan to establish safe zones in Syria and help end the six-year conflict.