US officials announced Tuesday that Trump had signed off on a plan "to equip Kurdish elements of the Syrian Democratic Forces" in the fight to retake Raqqa from the Islamic State in Iraq and Syria.
The dispute could ignite more fighting between the two key USA allies in the battle against the Islamic State group as Syrian Kurdish forces gear up for a major operation to drive the militants from their declared capital, Raqqa.
The decision infuriated Turkey, a fellow North Atlantic Treaty Organisation member, which regards the YPG as the Syrian arm of the Kurdistan Workers' Party (PKK) that has waged a deadly insurgency inside Turkey since 1984.
Turkey, the European Union, and the United States all consider the PKK a terrorist group.
Turkish media are quoting Deputy Prime Minister Nurettin Canikli as saying the USA decision to arm Syrian Kurdish fighters viewed as terrorists by Turkey is "unacceptable".
But from Turkey's perspective, the US decision amounted to a "very serious crisis" in the relationship, said Ufuk Ulutas, the foreign policy director at the Foundation for Political, Economic and Social Research, a government-friendly think tank in Ankara. "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".
A U.S. official said that small arms, machine guns, construction equipment and armored vehicles were among the provisions.
US -backed Syrian Democratic Forces have defeated Islamic State fighters in the city of Tabqa and have seized Tabqa Dam, a key objective for the militias before a planned attack on the terror group's de-facto capital, Raqqa.
The Syrian government has censured and protested the USA and Turkish support for armed militants in Syria, and has vowed to resist any moves by foreign-sponsored militants to seize territories inside the Arab country.
The dispute over arming Syria s Kurds poisoned ties between the two North Atlantic Treaty Organisation allies under the administration of former president Barack Obama but Ankara had hoped for smoother ties under Trump. SDF spokesman Talal Silo attributed the Tabqa victory to "the sacrifices of the SDF's heroes" and "the full, unlimited support of the US-led worldwide coalition", Reuters reported.
There is no reality in the comments that a ground operation against Daesh ( ISIS) can only be successful with the YPG.
Both Washington and Brussels classify the PKK as a terror group but has never regarded the YPG as such.
Cavusoglu also said the United States should distinguish between the YPG and their Arab allies in the SDF alliance, and that the Arabs should be the ones to enter Raqqa.
Mr Yildirim said he could not imagine the United States having to choose between Turkey's strategic partnership and a terrorist organisation. He said USA -led forces had "almost completely" isolated Raqqa and expected troops to soon begin advancing "within striking distance" of the city.
Speaking to reporters before departing for London on Wednesday, Yildirim said Turkey can not accept "direct or indirect" support for the Kurdish rebels, known as the PKK. "Our intent is to work with the Turks, alongside one another to take Raqa down", he said.