The Syrian army and its allies have launched a major attack on insurgents south of Aleppo, in what they say is the fiercest government assault in the area since an agreement to ease the fighting came into effect in February.
Fighting south of Aleppo in recent days has put further strain on the already widely violated ceasefire deal brokered by the United States and Russia with the aim of launching a diplomatic process towards ending the five-year-long war.
The indirect talks organised by the United Nations are struggling with no sign of compromise over the main issue dividing the sides: the future of President Bashar al-Assad.
A second round of talks is due to begin on Monday in Geneva.
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Rebels described intense air strikes in the southern Aleppo area where the al Qaeda-linked Nusra Front shot down a Syrian warplane on Tuesday and captured its pilot.
A statement from the Syrian army and its allies said they were targeting armed groups in the area, supported by "heavy and concentrated air strikes", the website of the Hezbollah-controlled al-Manar reported.
The statement said the attack was a response to insurgent violations of the cessation of hostilities agreement.
"This fire will continue until all the militants surrender," it said.
Syria's crisis erupted five years ago with protests against Assad that were put down with force.
It descended into a civil war, which has killed more than 250,000, drawn in global military powers and helped Islamic State establish its self-declared caliphate.
Nearly five million refugees have been driven abroad.
