Fourteen dead and dozens injured in 'terror' attack in Aleppo city
Ankara, which supports the rebels, has looked on with alarm at the YPG's advance, as it is fighting its own insurgency in Kurdish areas in its south-east and views the Syrian Kurdish paramilitaries as another wing of the Kurdistan Workers' party (PKK), with which it is fighting.
The Kurds believe that a coalition of armed opposition groups, which includes Ahrar ash-Sham, al-Jabha al-Shameea, Brigade of Sultan Myrad, Sultan Faith Battalions, Fa Istaqim Kama Omirt Batallions, Nour ad-Deen Zinki Battalions, 13th Brigade, al-Fau al-Oal (the first regiment), 166th Battalion, and Abu Omara Battalions, was responsible for the chemical attack.
The Observatory also reported that the regions of Syria covered by the cessation of hostilities enjoyed the "calmest day" since the deal started, a monitor said.
The residential area has been a major frontline in Aleppo for weeks, with heavy clashes between the Syrian Kurdish YPG militias and rebels in control of large parts of the eastern portion of the divided city.
The Observatory said the firing was by insurgents including the Al-Qaeda-linked Al Nusra Front.
Peace talks are scheduled to recommence on March 14 after the success of the ceasefire which has largely held since February 27.
The Observatory said it was the biggest single toll since the agreement came into effect.
The YPG is backed by the United States to fight Islamic State in Syria's northeast, and has been the most effective partner on the ground for a US-led air campaign against IS in Syria.
The YPG denies it is coordinating with the Syrian army.
Since Russia's intervention in the Syrian conflict, government forces and their allies have been recapturing territory lost past year in many parts of western Syria, including Aleppo province.
"The HNC has not taken a decision yet", Monzer Makhos said.
Earlier this week, the Russian Defense Ministry chided Turkey for ceasefire violations in the Syrian provinces of Aleppo and Idlib. "We are waiting for progress on the humanitarian issue and respect for the cease-fire".