Some 390,000 trapped civilians are afraid to venture out of their houses, and water, food and electricity supplies have run out.
The area is being hammered by one of the heaviest bombardments in seven years of war that has killed at least 250 people in 48 hours.
Syrian state media carried no reports of air strikes on Monday in Idlib but said the army and its allies had expanded their control around the air base.
The Syrian Observatory said on Friday around 70 rockets had fallen in 24 hours on Eastern Ghouta in the heaviest bombing since the de-escalation zone was declared.
The rebels warned on Saturday they would abandon the truce if government truce violations persisted.
Idea of storming country has been dropped, commentator says.
The YPG makes up a significant portion of the US-backed Syria Democratic Forces (SDF), a Kurdish and Arab alliance that fights against Islamic State insurgents in Syria.
Human Rights Watch interviewed five Palestinian minors whom the authorities arrested or detained in separate incidents for allegedly throwing rocks.
HRW report slams US and allies for allowing their military focus on fighting IS “to overshadow efforts to push Damascus to end its abuses.”
Activist group says the last month has been the bloodiest of the nearly three-year conflict.