Clashes around Syria's second city Aleppo have killed at least 16 pro-regime fighters and 19 members of Al-Qaeda's affiliate and allied rebel groups in 24 hours, a monitor said Sunday.
"Fierce fighting raged past midnight on several fronts in the south of Aleppo province," said the Syrian Observatory for Human Rights.
Hezbollah fighters were fighting alongside regime troops and other loyalist militia against jihadists and rebels, the monitoring group said.
"Shelling and fighting in the past 24 hours has left 19 Syrian and non-Syrian members of Al-Nusra dead… while 16 pro-regime fighters were also killed," the Observatory said, adding that one jihadist had blown himself up.
A truce brokered by the United States and Russia, which back opposing sides in Syria's war, does not apply to the fight against jihadists.
Across much of Syria, the February 27 truce has largely held. In areas where Al-Nusra fights alongside allied rebel groups, violence has been frequent despite the ceasefire.
Around Aleppo in particular, "the ceasefire has all but collapsed on the main front lines", Observatory director Rami Abdel Rahman said.
"Al-Nusra, (Islamist rebel group) Ahrar al-Sham and their allies are pushing an offensive to take back turf seized around Aleppo by the regime before the truce came into force," he told AFP.
Inside the city, the Observatory said regime barrel bomb strikes on Sunday hit the northeastern district of Al-Haidariyah, injuring a number of people including children.
"This is a new violation of the ceasefire inside the city," Abdel Rahman said.
Earlier on Sunday, Syria's Prime Minister Wael al-Halqi said regime troops were preparing a major operation to retake control of Aleppo with Russian air cover.
He said that regaining control of Aleppo from rebels hostile to the regime of President Bashar Al-Assad would allow government forces to advance east to Deir Ezzor, some 60% of which is under the control of the Islamic State (ISIS) group.
Syria's conflict erupted in March 2011 with anti-government protests but has since evolved into a multi-front war drawing in regional powers.
AFP contributed to this report.