At least 33 pro-regime fighters were killed Sunday in attacks mounted by jihadist groups near Idlib province, in the deadliest day in six months for loyalist forces, a monitor said.
Twenty-seven fighters were killed in two attacks by Ansar al-Tawhid jihadists, the Syrian Observatory for Human Rights said.
"Five jihadists were also killed," said Rami Abdel Rahman, director of the Britain-based monitoring group.
Ansar al-Tawhid has ties to the larger Hurras al-Deen group, which is also active in the area of northwest Syria. Both are considered semi-officials franchises of Al-Qaeda in Syria.
The area of Idlib and small parts of the adjacent provinces of Hama and Aleppo are mostly controlled by the rival Hayat Tahrir al-Sham organisation.
HTS is led by fighters who formerly belonged to Al-Qaeda's ex-affiliate in Syria.
The attacks by Ansar al-Tawhid were carried out against regime positions in Masasna, a village in Hama province, the Observatory said.
A military source quoted by state news agency SANA confirmed soldiers had been killed and wounded in the attacks on their positions near Idlib province.
Loyalists forces had killed some assailants, the source added without giving figures.
The foreign ministry said Syria "will not allow terrorists and those who are behind them to carry on with their attacks against innocent civilians and the armed forces".
In Latakia province, also northwest of the capital, at least six pro-regime fighters were killed in attacks by HTS militants later on Sunday, the Observatory said.
According to Abdel Rahman, the latest spate of attacks caused "one of the highest casualty figures among regime ranks since the Putin-Erdogan deal".
He was referring to an accord struck in the Russian resort of Sochi by Russian President Vladimir Putin and his Turkish counterpart Recep Tayyip Erdogan.
Under the September 17 truce deal, Turkey would exert its influence over anti-regime groups in the Idlib region to have them pull back fighters and heavy weapons from a demilitarised zone.
It was meant to stave off a planned offensive by the regime and its Russian backers that aid groups feared could spark the eight-year-old Syrian conflict's worst humanitarian crisis to date.
The government assault on the last major bastion of rebel forces has been held off but the deal's provisions have not been implemented.
Since the Sochi agreement, HTS has consolidated its grip on Idlib province.