Turkey's president vowed "to purge terrorism" across the border in Syria, as Turkish forces pounded US-backed fighters with artillery fire on Sunday.

A military operation in northern Syria against the city of Afrin - controlled by the Syrian Kurdish armed group YPG - will be launched "in the days ahead", President Recep Tayyip Erdogan was quoted as saying by the official Anadolu news agency.

He said the attack on Afrin would be an extension of the 2016 Euphrates Shield Operation, which targeted Islamic State of Iraq and the Levant (ISIL, also known as ISIS) fighters, as well as the YPG. The eight-month combat effort officially ended in March 2017.

Turkish soldiers are currently based in rebel-held territory on both sides of Afrin.

"In the coming days, God willing, we will continue with the Afrin [operation] that we started first with Euphrates Shield Operation to purge terrorism from our southern borders," Erdogan said in a speech in central Anatolian Tokat province.

The US views the YPG as the most effective fighting force against ISIL.

US President Donald Trump decided to arm YPG fighters, despite Turkey's objections and a direct appeal from Erdogan at a White House meeting last May.

American arms shipments began before the offensive to recapture the city of Raqqa from ISIL. YPG played a prominent role in defeating the group in its former de facto capital in northern Syria late last year.

Tensions between the NATO allies remain high, despite Trump saying last November that Washington would no longer supply weapons to the YPG.

Without mentioning the US, Erdogan said on Sunday he expected allies not to make "mistakes" by "taking sides" with the YPG during the battle for Afrin.

"We expect from our allies that they behave in accordance with the spirit of our deep-rooted relationship during this process," Erdogan said, adding "despite everything" he hoped for cooperation to achieve "common interests" in the region.

"I hope these forces will not make a mistake of appearing on the same stage with the terrorist organisation during the Afrin operation," the president said.

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A senior Syrian Kurdish official said on Sunday that fighting between the YPG and Turkish forces was already under way.

"There is attacks and clashes on the border between Turkey and the People's Protection Units YPG," Hediye Yusuf said on Twitter.

She called Turkey's operation against Afrin a "violation" that "undermines international efforts to reach a political solution in Syria".

Afrin canton faces an attack by Turkey, and there is attacks and clashes on the border between Turkey and the People's Protection Units YPG.

Afrin never hesitated to support Kobane, Reqqa, Manbej and Der Alzor by fighters to eliminate terrorism and made thousands of martyrs in th — Hediye Yusuf (@CudiHerjin) January 14, 2018

A YPG spokesman in Afrin told The Associated Press news agency fighting broke out after midnight between his unit and Turkish troops near the border.

Rojhat Roj said the shelling of areas in Afrin district, in Aleppo province, killed one YPG fighter and wounded several civilians on Sunday.

YPG will fight to "defend our gains, our territories", Roj said.

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The US-led coalition fighting in Syria has announced it will train about 15,000 Syrian Kurd fighters to be part of a 30,000-strong border force in the country's north.

"The base of the new force is essentially a realignment of approximately 15,000 members of the Syrian Democratic Forces [SDF] to a new mission in the Border Security Force, as their actions against ISIS draw to a close," Colonel Thomas F Veale, a coalition spokesman, told The Defense Post in a story published on Saturday.

The SDF, also consisting of Arab fighters, is dominated by the Kurdish YPG.

Turkey reacted angrily to the news on Sunday.

"Rather than end its support … these steps taken to legitimise a terror organisation and to make it permanent in the region are worrying," Ibrahim Kalin, Erdogan's spokesman, was quoted as saying by local media. "Accepting this state of affairs is absolutely not possible."

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Since December, Ankara has reinforced its southern border by sending armoured vehicles, tanks, and heavy machine guns, the Hurriyet daily cited sources as saying.

Turkey has been working closely with Russia and Iran to end the nearly seven-year Syrian war, despite Moscow and Tehran supporting Syrian President Bashar al-Assad - and Ankara backing the anti-Assad opposition.

YPG is considered by Turkey to be a "terrorist group" with ties to the banned Kurdistan Workers' Party (PKK), which has waged a decades-long fight inside the country.

PKK is blacklisted as a terrorist organisation by Turkey and its Western allies. More than 40,000 people in Turkey have been killed since the 1980s after the PKK launched its rebellion.