President Recep Tayyip Erdogan on Sunday threatened to attack the Kurdish militia-held town of Afrin in northern Syria "in the days ahead" to clear it of "terrorists".

"We will continue our operations begun with Operation Euphrates Shield to clean our southern borders of terror in Afrin (northern Syria) in the days ahead god willing," Erdogan said in a televised speech.

"The slightest disturbance on the border would be the signal for us to take a step."

The president referred to Turkey's previous eight-month military operation launched in August 2016 against the Islamic State (IS) extremist group and the Syrian Kurdish Peoples' Protection Units (YPG) militia. It ended in March last year.

Late last year, Turkish troops were then deployed to rebel-held northern Idlib province, south of Afrin, as part of an agreement with Iran and Russia to implement four so-called de-escalation zones in flashpoint areas around Syria.

Erdogan has repeatedly said that Afrin should be cleared of "terrorists" and in November 2016, he said Turkish troops needed to be deployed there.

Afrin is controlled by YPG militia considered by Ankara to be a terror group linked to the outlawed Kurdistan Workers' Party (PKK) waging an insurgency inside Turkey.

The PKK is blacklisted as a terror group by Ankara and its Western allies.

The US sees the YPG as the most effective fighting force against IS and last year provided it with arms ahead of major battles in Syria.

The issue is among many causing tense relations between Ankara and Washington, though Turkish officials said in November that US President Donald Trump apparently told them Washington would no longer supply weapons to the YPG.

"I hope that during an Afrin operation, these powers will not make the mistake of appearing to be on the same side as a terror organisation," Erdogan said in an apparent reference to the US during the rally in the northern Turkish city of Tokat.

He added he hoped Turkey "would take action together" with its allies.

Since December, Ankara has reinforced its southern border in Hatay and sent armoured vehicles, tanks and howitzers, sources told Hurriyet daily.

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