Syrian regime and Russian jets pounded on Saturday the northwestern province of Idlib a day after the failure of a three-way summit on the Syrian conflict.
More than 60 air raids killed at least four civilians in southern Idlib, according to the Syrian Observatory for Human Rights and rescue workers.
The renewed strikes came a day after the leaders of opposition backer Turkey and regime allies Russia and Iran failed at the summit in Tehran to agree on how to stave off a threatened offensive on the Idlib.
Aid organizations have warned that any military campaign to retake the region of nearly three million people on the Turkish border could spark one of the worst humanitarian disasters in Syria's seven-year war.
The Britain-based Observatory reported 68 air raids and 19 barrel bombs dropped Saturday on several of towns and villages in southwestern Idlib and Hama province. At least four civilians including two children were killed.
The Western-sponsored Syrian Civil Defense rescue service known as the White Helmets said they pulled four bodies, including a child, from the rubble of a building bombed by Russian planes in the village of Abdeen, near Khan Sheikhoun.
Russia says it avoids civilians and only targets radical al Qaeda inspired groups but opposition sources and residents say most of the casualties in the last few days were civilians.
An AFP correspondent in the southern village of Al-Muntar saw huge clouds of grey smoke billow up above olive trees after a barrel bombing.
Women and children ran terrified across nearby fields, one woman appearing to clutch a baby in a blanket.
An elderly man sat out of breath, barefoot and dazed in the red earth, his walking cane by his side.
The raids prompted hundreds of families to take to the roads, as dozens of cars and trucks tried to ferry civilians away from the bombardment.
Observatory head Rami Abdel Rahman said the raids targeted extremists and opposition positions, some of which were empty and others in use.
It was the heaviest bombardment since August 10, when at least 53 civilians were killed in Idlib and the neighboring province of Aleppo, he said.
Idlib is largely controlled by Hayat Tahrir al-Sham and rival factions.
The United Nations has warned that any offensive could force up to 800,000 people to flee their homes and urged key powerbrokers to avoid a "bloodbath".
On Friday, Turkey's President Recep Tayyip Erdogan openly disagreed with his Russian counterpart Vladimir Putin at a press conference in the Iranian capital.
Erdogan called for a "truce", while Russia's president said Damascus "has a right and must eventually take under control all of its national territory".
Iranian President Hassan Rouhani warned against a "scorched earth" policy, but said "fighting terrorism in Idlib is an unavoidable".