Turkey on Friday started fresh shelling of the Syrian town of Afrin in a move to oust a US-backed Kurdish militia that Ankara considers "terrorists" and vowed to press on with a full-scale operation against them.

The Turkish government has repeatedly warned it will strike Syrian towns controlled by the Kurdish People's Protection Units (YPG) militia, including Afrin, after the US said it was training a 30,000-strong border force there.

"The Afrin operation will take place," Defence Minister Nurettin Canikli told A Haber television.

"The presence of all the terror lines in northern Syria will be removed. There's no other way out," he said.

Turkish troops fired on several YPG targets in Afrin to prevent the formation of a "terror corridor" on the border, the state-run Anadolu news agency reported.

Army howitzers in the frontier Hatay province launched at least 10 rounds of artillery fire, targeting the "terror nests of the terror organisation in Afrin," Anadolu said.

A military convoy of 20 buses carrying Syrian opposition rebels backed by Ankara also crossed over into Syria through the Oncupinar border crossing in the Kilis province, Turkish media reported.

Separately, around 30 buses full of Syrian fighters headed towards the Cilvegozu border crossing in the town of Reyhanli, an AFP photographer said.

- 'De facto start'-

Canikli said with the shelling "in fact, the operation has de facto started."

Asked about the timing of a ground incursion, Canikli said: "It could be tomorrow, it could be in the evening. What we say is that this operation will take place."

Syria's deputy foreign minister Faisal Mekdad warned on Thursday that the Syrian air force could destroy any Turkish warplanes used in a threatened assault on the war-torn country.

The YPG is a major bone of contention in ties between Turkey and the US which considers it a key ally in fighting IS.

President Recep Tayyip Erdogan had reacted with fury to the announcement of the US-backed border force, denouncing it as an "army of terror".

The Pentagon said it does not plan to create an "army" and that the force is aimed at fighters from the Islamic State group and maintaining stability in areas recaptured from the jihadists.

Ankara however said it was not satisfied with the US assurances.

Turkey accuses the YPG of being a branch of the outlawed Kurdistan Workers' Party (PKK) that has waged an insurgency in its southeast since 1984.

Meanwhile, mortar fire on the Syrian town of Azaz just across the border from Turkey and held by Turkish-backed rebels wounded at least 14 people in a psychiatric hospital, a monitor said on Friday.

The Syrian Observatory for Human Rights, a Britain-based monitor of the war, said the mortar rounds on Thursday were fired by the Syrian Democratic Forces, a US-backed alliance dominated by the YPG.

The Turkish army condemned the mortar fire and said wounded civilians were also taken across the border into Turkey for treatment.

Analysts say Turkey needs the green light from Russia for a full cross-border operation because of Moscow's military presence in the area.

In a surprise development, Turkey's army chief General Hulusi Akar and spy chief Hakan Fidan were in Moscow on Thursday for talks with Russian counterparts on security issues and Syria.