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 within a 24-hour period, a monitor said Sunday.
"Fierce fighting raged past midnight (Sunday) on several fronts in the south of Aleppo province," said the Syrian Observatory for Human Rights.
Lebanese Shiite 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 of the jihadists 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 in spite of 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.
The civilian death toll has dropped as air raids and barrel bomb strikes by the regime on residential areas have all but ceased, Abdel Rahman said.
"The violence around front lines has by no means stopped," he added.
Furthermore, rebels have frequently shelled Sheikh Maqsud, a mainly Kurdish area of Aleppo, leaving dozens of civilians dead since the truce began, he said.
Syria's conflict erupted in March 2011 with anti-government protests but has since evolved into a multi-front war drawing in regional powers.