Ankara launched a cross-border attack against Syria's Kurds on October 9.

US forces withdrew from their largest base in northern Syria on Sunday, a monitor said, as Turkey accused Kurdish forces of killing one of its soldiers in fighting that tested a fragile truce.

Ankara launched a cross-border attack against Syria's Kurds on October 9 after the United States announced a military pullout from the north of the war-torn country.

A US-brokered ceasefire was announced late Thursday, giving Kurdish forces until Tuesday evening to withdraw from a "safe zone" Ankara wants to create along its southern frontier.

The Kurds have been a key ally to Washington in the US-backed fight against Islamic State group jihadists in Syria, but Turkey views them as "terrorists" linked to Kurdish militants on its own soil.

On Sunday, an AFP correspondent saw more than 70 US armoured vehicles escorted by helicopters drive past the northern Syrian town of Tal Tamr carrying military equipment.

Some flew the American stars-and-stripes flag as they made their way eastwards along a highway crossing the town, he said.

The Syrian Observatory for the Human Rights said the convoy was evacuating the Sarrin military base.

The vehicles appeared to be heading to the town of Hassakeh, further east, said the Britain-based monitor, which relies on sources inside Syria for its information.

Sarrin is situated more than 30 kilometres (19 miles) south of the border town of Kobane, on the edges of the Turkish-desired "safe zone," Observatory chief Rami Abdel Rahman said.

- Cigarettes and motorbikes -

Sunday's pullout was the fourth such withdrawal of American forces in a week and left Syria's northern provinces of Aleppo and Raqa devoid of US troops, Abdel Rahman said.

Turkey and the Kurds have traded accusations that the other was not abiding by the deal brokered by US Vice President Mike Pence.

On Sunday, Turkish Foreign Minister Mevlut Cavusoglu said Ankara had seen some Kurdish withdrawals.

"We on the one side have observed some withdrawals, but on the other side the group is engaged in provocations toward us," he said in an interview with Turkish television Kanal 7.

The Turkish defence ministry said one of its troops was killed by Kurdish forces in the area around the Turkey-held Syrian border town of Tal Abyad.

"One of our heroic comrades fell martyr and another was wounded after anti-tank and small arms fire by... terrorists during their reconnaissance and surveillance mission," it said, referring to Kurdish fighters.

Turkish forces returned fire in self-defence, it added.

The top figure on the Kurdish side, Syrian Democratic Forces (SDF) commander Mazloum Abdi, has said Turkey was blocking his forces' withdrawal and trying to blame any collapse of the deal on the Kurds.

The Observatory said sporadic clashes were ongoing in the north and northwest of Ras al-Ain, a border town surrounded by Turkish troops and their Syrian proxies.

An AFP reporter said empty neighbourhoods in the northeast of the town were calm.

He saw Turkey-backed fighters playing around on motorbikes and sitting on the pavement smoking cigarettes.

- Anti-IS operations -

On Saturday, Abdi said the US was not doing enough to force Ankara to abide by the agreement.

"If there is no commitment, we shall consider what happened a game between the Americans and Turkey -- on one side preventing the troop withdrawal while on the other claiming our forces did not withdraw," he said.

He however said that his forces had resumed working with the US-led coalition against IS in the east of the country, and insisted Washington's presence in the country was important.

"We have restarted military action against IS cells in Deir Ezzor. Our forces are working there with the coalition forces," he told AFP.

A week ago, the Pentagon said US President Donald Trump had ordered the withdrawal of up to 1,000 troops from northern Syria as Turkish troops advanced into Syrian territory.

The violence since October 9 has killed hundreds from both sides and displaced hundreds of thousands from their homes, the Observatory says, in the latest humanitarian crisis in Syria's eight-year civil war.

The international community has warned that the incursion could draw Kurdish fighters away from jails and camps where they are guarding thousands of suspected IS fighters and family members.

They fear a resurgence of the extremist group, which the SDF expelled from the last scrap of its self-proclaimed "caliphate" in March.

The jihadists have however continued to claim responsibility for deadly attacks in SDF-held territory.