Syria's White Helmets say at least six people were killed and 10 remain under rubble following government air raids on a town in the rebel's last stronghold.
The first responders known as White Helmets said Monday that rescue workers were still searching for survivors under the rubble after the airstrikes hit a crowded residential street in the town of Ariha. They said five women and a child were killed.
The airstrikes hit the town amid intense government bombardment in Idlib province and its environs.
The Syrian Observatory for Human Rights says there were more than 100 air raids and as many as 93 barrel bombs dropped Monday on the southern section of the rebel stronghold.
The violence has displaced more than 200,000 people since erupting in late April.
Russian-backed Syrian government forces will be able to advance all the way to the Turkish border if they pierce rebel defences in the northwest, a top opposition official said, urging Turkey to do more to shield the area from a major attack.
The month-long onslaught is the most serious escalation of the war between President Bashar al-Assad and his enemies since last summer. Syrian government air strikes and barrel bombing backed by Russian air power have uprooted around 250,000 people in the territory, the last significant rebel stronghold.
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Fawaz Hilal, head of the "Salvation Government" that runs Idlib province, expressed confidence that opposition fighters gathered in the Idlib region from all over Syria would be able resist the onslaught.
"This ferocious attack is a bone-breaking battle. If the regime is able to break our defensive lines in northern Hama and southern Idlib it will not stop until it reaches the borders," Hilal told Reuters in an interview.
His government, backed by the powerful Tahrir al-Sham jihadist group, had called on its employees to help shoulder the "military burden" through building sandbag defences, manning front lines, financial support or any other help.
"We are all concerned with repelling this attack," he said.
The bombardment has killed 229 civilians and injured 727 since April 28, according to The Union of Medical Care and Relief Organisations (UOSSM), a U.S.-based medical NGO.