The bombing ripped through Tal Abyad, one of several once Kurdish-controlled towns seized by Turkey last month in a deadly cross-border offensive.

The blast came despite a truce last week to halt a Turkish assault that began on October 9 and sparked the latest humanitarian disaster of Syria's eight-year civil war.

On Saturday, an AFP correspondent in Tal Abyad saw the skeletons of two motorbikes ablaze in the middle of a rubble-strewn street.

A group of men carried the severely burnt body of a victim onto the back of a pickup truck, as a veiled young woman stood aghast by the side of the street.

Turkey's defence ministry said 13 civilians were killed in the attack, which it blamed on Kurdish fighters.

The Syrian Observatory for Human Rights, a Britain-based war monitor, reported 14 people -- pro-Ankara fighters and civilians -- had been killed in the explosion.

Meanwhile, in the Kurdish-majority city of Qamishli, thousands of Syrian Kurds marched in the streets to protest what they view as a Turkish invasion.

"No to Turkish occupation," they cried, brandishing flags of their once semi-autonomous region and its fighters.

US troops return?

The truce deal signed last week between Ankara and Moscow to end the Turkish offensive demands Kurdish fighters withdraw from the border.

It hands a 120-kilometre-long (70-mile-long) stretch of border land including Tal Abyad over to Turkey, and provides for joint Russian-Turkish patrols along other parts of the frontier.

The first of those started on Friday.

Ankara views Syrian Kurdish fighters as "terrorists", and wants to expel them from areas along its southern border.

But Turkey also hopes to resettle there some of the 3.6 million Syrian refugees it hosts on its own soil.

On Friday, UN Secretary General Antonio Guterres told Turkey's President Recep Tayyip Erdogan the United Nations would study Ankara's plans for repatriation.

On Saturday, US troops visited Kurdish forces in Qamishli in the second such spotting of American forces in northeast Syria since a shock pullout announcement last month triggered the Turkish attack.

Beige-coloured armoured vehicles flying the American flag pulled up at the headquarters of the Kurdish-led Syrian Democratic Forces, the de-facto army of Syria's Kurds.

The SDF have been a key US ally in fighting the Islamic State jihadist group, backed by air strikes by a US-led coalition.

A source who attended a meeting with the Americans on Saturday said they wanted to return to set up a military post in Qamishli.

The coalition declined to comment specifically on Saturday's visit, but said the alliance continued to withdraw forces from northern Syria, redeploying some troops to oil-rich eastern Syria.

'No confidence'

The US withdrawal was largely seen as a betrayal of its Kurdish allies, who were forced to seek help from the Russia-backed Damascus regime to contain the Turkish attack.

In an interview published Saturday, SDF commander-in-chief Mazloum Abdi said he distrusted both the Syrian government and Russia, but had no other choice but to work with them.

"We have no confidence, but it's not possible to solve Syria's problems without using the political path. We must negotiate," he told Italy's La Repubblica newspaper.

The SDF expelled IS jihadists from their last patch of territory in Syria in March.

But the extremists continue to claim deadly attacks in SDF-held areas, and this week they announced they had a new leader after their former chief Abu Bakr al-Baghdadi was killed in a US raid.

After years of war against IS, the SDF guard around 12,000 suspected jihadist fighters in overcrowded jails.

The Turkish offensive last month sparked international alarm that extremists would escape from jail and regroup, with the United States admitting around 100 had already taken flight.

Syria's war has spiralled into a complex conflict involving word powers since it started in 2011 with the brutal repression of anti-government protests.