Beirut - Rebels and extremists traded fire with government forces in northern Syria overnight, their “fiercest” exchanges since a buffer zone deal was announced for the area last month, a monitor said Thursday.

A 15-20km wide “de-militarised zone” was announced by rebel backer Turkey and regime ally Moscow on September 17 to separate regime troops from rebel fighters in their last major bastion in Idlib province and adjacent areas.

Shelling has continued intermittently, however, and escalated dramatically late Wednesday, the Syrian Observatory for Human Rights said.

“It was the fiercest bombing yet since September 17,” said Rami Abdul Rahman, who heads the Britain-based monitoring group.

“Syrian regime rocket and artillery fire killed one girl in Kafr Hamra,” an Aleppo province town inside the declared buffer zone.

Rebel shelling from inside the zone killed three civilians in regime-held territory earlier this week.

Late on Wednesday, rocket fire by both extremists and Turkish-backed rebels hit second city Aleppo, wounding 10 people, Abdul Rahman said.

Regime news agency SANA gave the same casualty toll and said the regime’s army responded against the sources of the fire north and northwest of the city.

The National Liberation Front, the Turkish-backed rebel alliance which is the main armed group in that area, said it was responding to regime violations of the truce deal with “light and medium weapons.”

Under the deal agreed by Russia and Turkey, the rebels were supposed to have removed all heavy weapons from the buffer zone by October 10.

“Radical” fighters - taken to mean Hayat Tahrir Al Sham (HTS), the extremist alliance which is the dominant faction in Idlib, and other hardliners - were supposed to leave the zone by October 15.

But 10 days on there is still no sign of any pullout or of the planned monitoring patrols by the deal’s co-sponsors, the Observatory said.

Syrian pro-regime newspaper Al Watan said the extremists’ failure to withdraw “provides the justification for the Syrian army and Russian air force to start a military operation to oust (HTS) from the area.”

But both Russia and Turkey have said the truce deal remains on course despite the missed deadline, and their leaders are to be joined by their French and German counterparts for a four-way summit on Syria in Istanbul on Saturday.