A Syrian militant group linked to Al Qaeda killed 21 regime and allied forces on Sunday near Idlib province, in one of the most serious breaches of a six-month-old truce deal.
"At dawn, 21 fighters from the regime forces or allied militia were killed in an attack by Ansar Al Tawhid," the Syrian Observatory for Human Rights said.
"Five militants were also killed," said Rami Abdulrahman, the director of the monitoring group in Britain.
Ansar Al Tawhid has ties to the larger Hurras Al Deen group, which is also active in the area. Both are considered semi-official franchises of Al Qaeda in Syria.
Idlib and small parts of the adjacent provinces of Hama and Aleppo are mostly controlled by the rival organisation Hayat Tahrir Al Sham.
Hayat Tahrir is led by fighters who belonged to Al Qaeda's former affiliate in Syria, Jabhat Al Nusra.
Sunday's deadly attack was carried out against regime positions in the village of Masasna, in the north of Hama province, the Observatory said.
"It was one of the highest casualty figures among regime ranks since the Putin-Erdogan deal," Mr Abdulahman said.
He was referring to an agreement reached in the Russian resort of Sochi between Russian President Vladimir Putin and Turkish leader Recep Tayyip Erdogan.
Under the September 17 deal, Turkey would exert its influence over anti-regime groups in the Idlib region to get them to pull back their fighters and heavy weapons from a demilitarised zone.
The agreement was meant to stave off a planned offensive by the regime and its Russian backers, which aid groups feared could spark the eight-year Syrian conflict's worst humanitarian crisis yet.
The government assault on the last major bastion of forces opposed to President Bashar Al Assad's rule has been held off but the deal's provisions have not been implemented and the truce looks shakier than ever.
Since the Sochi agreement, Hayat Tahrir has consolidated its grip on Idlib and Turkey appears to be in no position to deliver on its commitment.
Breaches of the demilitarised zone have increased sharply in recent days. Another 20 regime and allied forces were killed in three days of clashes about a week ago.
