At least 19 civilians were killed during two separate incidents in the latest flare-up of violence along Syria's northern border with Turkey, a war monitor said on Friday.

The bloodshed comes against a backdrop of increased tensions pitting Kurdish-led Syrian Democratic Forces backed by the regime against Turkish forces and their Syrian proxies.

In one incident, the British-based Syrian Observatory for Human Rights said artillery fire by the regime of President Bashar al-Assad on the border town of Al-Bab, which is under the authority of Turkish-controlled Syrian forces, killed 15 civilians on Friday morning.

"Children were among the victims and at least 40 others were wounded," said the Observatory which has a network of sources in Syria. It added that the shelling originated from regime positions.

The monitor had earlier given a toll of nine dead and around 30 wounded.

A spokesman for the Kurdish-dominated SDF denied any involvement.

The shelling struck a busy marketplace, where residents and first responders rushed to assist and evacuate the wounded.

An AFP correspondent said the strike had ripped through the market area and described a jumble of body parts, strewn vegetables and mangled handcarts.

Violence between Turkey and Syria's Kurds escalated this week with a deadly Turkish strike killing 17 regime and Kurdish fighters in retaliation for Kurdish fire inside Turkey.

On Friday, the semi-autonomous Kurdish adminstration in northeastern Syria said an overnight Turkish strike hit a rehabilitation centre for girls in a region near the city of Hasakeh.

The Observatory said four children enrolled in the centre were killed and 11 were wounded.

Ankara considers the main Kurdish component of the SDF -- allied with the United States against Islamic State jihadists -- to be a terrorist organisation with links to the Kurdish insurgency inside its own territory.

Turkish President Recep Tayyip Erdogan has renewed threats of a broad operation against the Kurds in northern Syria. A July 19 summit with Iran and Russia, the two other major foreign brokers in the area, failed to get their endorsement of a fresh offensive against the Kurdish fighters.