The Kurdish Peoples' Protection Units (YPG) is seen by Washington as the best ally against Islamic State (IS) jihadists in Syria and the prime attacking force in any assault on their stronghold of Raqa

President Recep Tayyip Erdogan on Wednesday urged the United States to reverse a decision on arming Syrian Kurdish fighters whom Turkey considers terrorists, saying he would raise the issue in talks with Donald Trump next week.

The Kurdish Peoples' Protection Units (YPG) militia applauded a "historic" move that would speed up the defeat of Islamic State (IS) extremists but the issue risks stoking tensions between the two NATO allies ahead of Erdogan's critical visit to Washington.

US forces, accompanied by Kurdish People's Protection Units (YPG) fighters, drive armoured vehicles near the northern Syrian village of Darbasiyah, on the border with Turkey on April 28, 2017

The YPG is seen by Washington as the best ally against Islamic State group jihadists in Syria and the prime attacking force in any upcoming assault on their stronghold of Raqa.

But Ankara regards the YPG 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.

"I hope very much that this mistake will be reversed immediately," Erdogan said in Ankara alongside Sierra Leone counterpart Ernest Bai Koroma.

"We want to believe that our allies would prefer be side by side with ourselves rather than with the terror groups," Turkish President Recep Tayyip Erdogan said

"I will personally express our worries in a detailed way when we talk with President Trump on May 16," he added, saying the issue would also be discussed at the NATO summit in Brussels on May 25.

"We want to believe that our allies would prefer be side by side with ourselves rather than with the terror groups," he added.

Erdogan's statement capped a day of alarmed comments by Turkish officials, with Foreign Minister Mevlut Cavusoglu saying that "every weapon that turns up in their hands is a threat directed toward Turkey".

- 'Informed in advance' -

Pentagon chief Jim Mattis sought to allay Turkish concerns, saying at a news conference in Lithuania the US would work very closely with Turkey over security on its border with Syria.

"We have very open discussions about options and we will work together, we will work out any of the concerns," he said.

"I will personally express our worries in a detailed way when we talk with President Trump on May 16," Turkish President Erdogan said

In a surprise announcement, the Pentagon had 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 YPG hailed the move as "historic" and said it would now play "a more influential, powerful, and decisive role" in fighting IS.

YPG spokesman Redur Xelil described the move as "somewhat late", but would still "provide a strong impetus" to all forces fighting IS.

The SDF, a US-backed group dominated by YPG but which also contains Arab elements, said that receiving US arms and military equipment would "hasten the defeat" of the jihadists.

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.

While the government expressed predictable anger, the deputy head of the opposition Republican People's Party (CHP) Ozturk Yilmaz said it should go even further by postponing Erdogan's visit to the US

A US officer, from the US-led coalition, speaks with a fighters from the Kurdish People's Protection Units (YPG) at the site of Turkish airstrikes near northeastern Syrian Kurdish town of Derik, known as al-Malikiyah in Arabic, on April 25, 2017

The timing of the announcement was especially delicate as a high-level Turkish delegation including Chief of Staff General Hulusi Akar, presidential spokesman Ibrahim Kalin and Turkey's spy chief Hakan Fidan had been in the US laying the groundwork for the meeting.

According to the New York Times, the delegation was informed of the decision to arm the YPG by Trump's national security adviser H.R. McMaster.

Turkish media said the three met McMaster at the White House on Monday but gave no details over the content of the talks.

Both Washington and Brussels classify the PKK as a terror group but do not regard the YPG as such.