Clashes between the Syrian rebels and regime forces are reported just days ahead of a planned four-way summit of leaders from Turkey, Russia, France and Germany on Syria in Istanbul on Saturday.
Syrian rebels traded fire with regime forces in northern Syria overnight, their "fiercest" exchanges since a demilitarised zone deal was announced for the area last month, a monitor said on Thursday.
A 15- to 20-kilometre (nine to 12-mile) wide "demilitarised zone" was announced by Turkey and Russia on September 17 to separate regime forces from rebels in their last major bastion in Idlib province and adjacent areas.
Shelling has continued intermittently, however, and escalated dramatically late on Wednesday, the Syrian Observatory for Human Rights said.
"It was the fiercest bombing yet since September 17," said Rami Abdel Rahman, who heads the Britain-based monitoring group.
He said Syrian regime rocket and artillery fire killed one girl in Kafr Hamra, an Aleppo province town inside the declared demilitarised zone.
Late on Wednesday, rocket fire by both the sides hit second city Aleppo, wounding 10 people, Rahman said.
Regime news agency SANA gave the same casualty toll and said regime forces have 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 rebel group in that area, said it was responding to regime's 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.
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 claimed.
A pro-regime newspaper Al-Watan said the rebels' failure to withdraw "provides the justification" for the Syrian regime forces and Russian air force to start a military operation.
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.