The sudden flare-up in the more than decade-old civil war in Syria just as a fragile ceasefire took effect in neighbouring Lebanon drew appeals for de-escalation from across the international community.

Islamist group Hayat Tahrir al-Sham (HTS) and its allies met much tougher resistance in the countryside north of Hama than they did in the Aleppo region on Friday and Saturday, a Britain-based war monitor said.

The Syrian Observatory for Human Rights reported the heaviest fighting with government forces so far as the offensive entered its seventh day.

"Clashes have erupted in the northern Hama countryside, where rebel factions managed to seize several cities and towns in the last few hours," the Observatory said.

"Syrian and Russian air forces carried out dozens of strikes on the area."

Syrian state media reported that the two air forces had bombed the rebels in their Idlib stronghold as well as their vanguard in Hama province.

Russia is a key ally of President Bashar al-Assad. Its 2015 intervention in the Syrian civil war turned the tide in its government's favour but since 2022 the Ukraine war has tied down much of its military resources.