The Syrian army and its allies launched a big attack overnight on insurgents south of Aleppo, described as the fiercest government assault in the area since a cessation of hostilities agreement came into effect in February.
The Syrian Observatory for Human Rights, a British-based organization that tracks the war, said air strikes, artillery and rockets were being used in an attack aimed at recovering the town of Telat al-Eis, captured by rebels in recent days.
The al Qaeda-linked Nusra Front shot down a Syrian warplane in the area on Tuesday, capturing its pilot.
A statement from the Syrian army and its allies said they had started to target the positions of armed groups in areas south of Aleppo, 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.
A rebel said it was the heaviest attack in the south Aleppo area since the start of the cessation of hostilities deal agreed by Russia and the United States. Hany al-Khaled of the Sham Revolutionary Brigades group, an affiliate of the Levant Front, said the attack had been repelled and Shi'ite militias fighting with the government had suffered heavy losses.
The Observatory said government forces had made progress but had not been able to recapture Telat al-Eis.