A Syria war monitor on Thursday said clashes between the army and Syrian armed groups killed more than 130 combatants in the worst fighting in the country's northwest in years, as the government also reported fierce battles.
The Britain-based Syrian Observatory for Human Rights said rebel armed group Hayat Tahrir al-Sham (HTS) and allied factions launched a surprise attack on the Syrian army in the northern province of Aleppo on Wednesday.
The toll "in battles ongoing for the past 24 hours has risen to 132, including 65 fighters from HTS", 18 from allied factions "and 49 members of regime forces", said the Observatory, which relies on a network of sources inside Syria.
Some of the clashes, in an area straddling Idlib and Aleppo provinces, are less than 10 kilometres (six miles) southwest of the outskirts of Aleppo city.
HTS, led by Al-Qaeda's former Syria branch, controls swathes of much of the northwest Idlib area and slivers of neighbouring Aleppo, Hama and Latakia provinces.
According to news agency AFP, heavy, uninterrupted clashes were reported to take place east of the city of Idlib since Wednesday morning, including air strikes.
A military statement carried by state news agency SANA said that "armed terrorist organisations grouped under so-called 'Nusra terrorist front' present in Aleppo and Idlib provinces launched a large, broad-fronted attack" on Wednesday morning.
It said the attack with "medium and heavy weapons targeted safe villages and towns and our military sites in those areas".
The army "in cooperation with friendly forces" confronted the attack "which is still continuing", inflicting "heavy losses" on the armed groups, the military statement said, without reporting army losses.
The Observatory said HTS was able to advance in Idlib province, taking control of Dadikh, Kafr Batikh and Sheikh Ali "after heavy clashes with the regime forces with Russian air cover".
"The villages have strategic importance due to their proximity to the M5 international highway", the monitor said, adding the factions, which already took control of two other locations, were "trying to cut the Aleppo-Damascus international highway".
The Observatory said that "Russian warplanes intensified air strikes", targeting the vicinity of Sarmin and other areas in Idlib province, alongside "heavy artillery shelling" and rocket fire.
Syria's conflict broke out after President Bashar al-Assad repressed anti-government protests in 2011, and spiralled into a complex conflict drawing in foreign armies and armed groups.
It has killed more than 500,000 people, displaced millions and battered the country's infrastructure and industry.
The Idlib region is subject to a ceasefire -- repeatedly violated but still largely holding -- brokered by Turkey and Damascus ally Russia after a Syrian government offensive in March 2020.