The Syrian Observatory for Human Rights and White Helmets Civil Defence say scores of people have been killed since the Assad regime and its Russian ally renewed violence in Idlib and Hama provinces 13 days ago.
Syrian regime forces are advancing further into the last area of the country controlled by opposition, rebels as well as militants.
The regime and its Russian ally unleashed a renewed wave of violence on April 30, using barrel bombs, ground forces and air strikes on the north-western region held by Syria's former Al Qaeda affiliate Hayat Tahrir al Sham (HTS).
The regime's ground operation is being supported by Russian air strikes in Idlib and Hama provinces, violating a buffer zone agreed upon by Moscow and Ankara in 2018.
Britain-based war monitor Syrian Observatory for Human Rights (SOHR) said the regime is now in control of a number of villages, forming an L-shape at the far southern corner of the territory.
The monitor said on Sunday there were 20 Russian air strikes, including barrel bombs being dropped by helicopters.
The Observatory said at least 299 people were killed since the April 30.
On Thursday, Syria Civil Defence or White Helmets tweeted that at least 155 people have been killed since the start of the assault.
The UN has warned of a humanitarian disaster if the fighting continues.
3) In the consecutive 13 days of the massacre: 22 children have been killed and 77 injured to varying degrees. The death toll is 122 with 329 injured by this campaign. #WhiteHelmets #Syria pic.twitter.com/vYZgSHlPG7— The White Helmets (@SyriaCivilDef) May 9, 2019
HTS chief calls for arming to defend Idlib
The head of Syria's former Al Qaeda affiliate in a video released Sunday urged supporters to "take up weapons" to defend his group's bastion of Idlib against increased bombardment.
"We call for anybody able to take up weapons... to head to the battlefield," HTS chief Abu Mohammed al Jolani. said.
The spike in violence against the Idlib region since late April signalled "the death of all previous agreements and conferences", he said during an interview with a local media activist in a countryside setting.
It shows that "only jihadists [militants] and military might can be relied on," said Jolani, wearing a camouflage uniform and sitting cross-legged in the grass.
The activist said the interview was recorded in the northern Hama countryside, although it was not immediately possible to verify this.
It appeared to have been made in recent days after a regime advance.
Idlib, a region of some three million people, is supposed to be protected from a massive regime assault by a September buffer zone deal signed by Russia and opposition-backer Turkey.
But that agreement was never fully implemented after militants refused to withdraw from the planned buffer area.
Damascus and its allies have taken control of several towns from the militants in recent days.
The uptick in air strikes and shelling has displaced 180,000 people between April 29 and May 9, the United Nations said.
Damascus has not announced a wide offensive, but instead regularly announces targeting "terrorist" – meaning militant or rebel – positions.
Analysts believe the offensive will be limited.
The regime and Russia have accused the militants of targeting the main Russian airbase of Hmeimim to the west of Idlib.
Russia intervened in the war in 2015, and has since helped the regime to regain large parts of the country.
The civil war in Syria has killed more than 370,000 people and displaced millions since it started with the repression of anti-regime protests in 2011.