Clashes in north-west Syria killed 22 pro-government fighters and militants on Sunday in the highest death toll since the start of a ceasefire there two months ago, a monitor said.
A truce since March 6 had largely stemmed fighting in Idlib, Syria's last major rebel bastion, after a months-long regime campaign in which hundreds of civilians were killed and almost a million were forced to flee.
But before dawn on Sunday, rebels attacked the positions of pro-regime fighters on the western flank of the militant-dominated region, according to anti-government monitor the Syrian Observatory for Human Rights.
The clashes in the Sahl Al Ghab area killed 15 regime fighters, as well as seven jihadists including members of the Al-Qaeda-affiliated Hurras Al Deen group, the UK-based monitor said.
"It's the highest death toll for fighters since the truce came into force," said Observatory chief Rami Abdel Rahman, who relies on sources inside Syria.
"There had been intermittent clashes and bombardment between both sides before, but this is the most violent attack yet."
The Idlib region of some three million people is dominated by the Hayat Tahrir Al Sham group led by Syria's former Al Qaeda affiliate, but other militants, such as Hurras Al Deen and rebel groups, are also present.
The truce brokered by Syria government ally Russia and rebel backer Turkey has kept Syrian and Russian warplanes out of the region's skies, and has largely held despite sporadic clashes or rocket fire.
Tens of thousands have returned to their hometowns.
Hundreds of thousands of others remain in crowded displacement camps or in temporary shelters near the Turkish border.
Aid groups have warned that any outbreak of the novel coronavirus there would be devastating.
Syria's war has killed more than 380,000 people and displaced millions since starting in 2011 with the repression of anti-government protests.