Turkish-led bombardment on Friday killed 14 civilians in northeastern Syria, as Turkey's president threatened to broaden his assault and an hours-old US-brokered deal already appeared to crumble.

A war monitor said Turkish air strikes and mortar fire by its Syrian proxies killed 14 civilians, appearing to dash the ceasefire announced late Thursday.

That deal was meant to provide a five-day pause for the evacuation of Kurdish fighters from the battleground border town of Ras al-Ain and other areas Turkey wants to control along its border with Syria.

“If the promises are kept until Tuesday evening, the safe zone issue will be resolved,” Erdogan told reporters in Istanbul.

“If it fails, the operation ... will start the minute 120 hours are over,” he said.

The suspension looked designed to help Turkey achieve its main territorial goals without fighting, but its Syrian proxies continued to clash with Kurdish fighters on Friday.

The 14 civilians were killed in Turkish airstrikes and mortar fire by allied Syrian fighters on and around the village of Bab al-Kheir, the Syrian Observatory for Human Rights said.

The Britain-based war monitor said eight fighters of the Syrian Democratic Forces — the de facto army of the embattled Kurdish autonomous region — were killed in the strikes.

SDF spokesman Mustefa Bali said Turkey was clearly violating the terms of the agreement reached during a Thursday visit to Ankara by US Vice President Mike Pence.

Violation

“Despite the agreement to halt the fighting, air and artillery attacks continue to target the positions of fighters, civilian settlements and the hospital” in Ras al-Ain, he said.

Under the deal, Kurdish forces are required to withdraw from a border strip 32 kilometres deep, clearing the way for a “safe zone” sought by Turkey.

The Kurdish-led SDF had said they were ready to abide by the ceasefire in border territory between Ras al-Ain and Tal Abyad to its west.

Kurdish forces have put up fierce resistance in Ras al-Ain, with a network of tunnels, berms and trenches that held off the Turkish onslaught for a week.

On Friday afternoon, an AFP correspondent on the Turkish side of the border saw a big column of black smoke rise from Ras al Ain, though it was unclear what was burning.

The Turkish offensive was sparked by US President Donald Trump's announcement of an American withdrawal from northern Syria, leading critics to accuse him of betraying Washington's Kurdish allies.

The Turkish military and its Syrian proxies — mostly Arab and Turkmen former rebels used as a ground force — have so far seized around 120 kilometres of territory along the border.

More than 500 people have been killed on the two sides, including nearly 100 civilians, while around 300,000 have been displaced, according to the Observatory.

Ankara considers the Kurdish forces to be “terrorists” linked to Kurdish rebels inside Turkey.

Turkish forces and their allies had taken control of half of Ras al-Ain on Thursday when its hospital was hit, trapping patients and staff inside, Abdel Rahman said.

Kurdish authorities sent a medical team to rescue the wounded but it was prevented from entering the town, said Hassan Amin, a director of the hospital in nearby Tal Tamr.

Seven wounded people from around Ras al-Ain made it to Tal Tamr, he said.

'War crimes'

Trump has come under criticism in Washington over his handling of the crisis, from Democrats and from within his own Republican Party.

The SDF fought alongside US forces to defeat the militant Islamic State (IS) group in Syria and Iraq, but Trump argued it was no longer the US role to ensure calm in the region.

French President Emmanuel Macron, whose country has special forces in Syria as part of the anti-IS coalition, complained he had learnt of the US withdrawal by Twitter.

Thousands of IS fighters and their family members are held in Kurdish-run jails and camps across northern Syria.

The prospect of thousands of the world's most radical jihadists breaking out in the chaos caused by Turkey's invasion is causing widespread alarm.

The Turkish offensive has also been widely criticised, with videos surfacing online allegedly showing captured fighters and civilians being executed.

The Kurds on Thursday accused their rivals of using banned weapons such as napalm and white phosphorus munitions, a charge Ankara has denied.

Amnesty International on Friday accused Ankara's forces and their proxies of “serious violations and war crimes, summary killings and unlawful attacks”.

There was no immediate response from Ankara, which says it takes all possible measures to avoid civilian casualties.

Trump welcomed the ceasefire on Thursday, but later compared the warring parties to children. “Like two kids in a lot, you have got to let them fight and then you pull them apart,” he said.