The Syrian army and its allies launched a big attack overnight on anti-Assad forces south of Aleppo, using air strikes, artillery and rockets, according to The Syrian Observatory for Human Rights, a British-based organization that monitors the war.

The government’s assault was described as the fiercest in the area since a cessation of hostilities agreement came into effect in February. It aimed at taking back control over the town of Telat al-Eis, seized by factions opposing Assad in recent days.

Jaish al-Fath (the army of conquest) which includes moderate and hardline opposition forces 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 other groups’ violations of the cessation of hostilities agreement.

One of the militants fighting against Assad 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 deterred 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.