Rebel sources said their fighters had taken three villages so far. The Syrian Observatory for Human Rights, which monitors the war, described it as a fierce attack "carried out by factions loyal to Ankara".
Syrian government forces, backed by Russian air power, have made rapid advances in Idlib this week, capturing the town of Maarat al-Numan which is located about 100 km southwest of al-Bab.
Idlib and the area north of Aleppo form part of the last major rebel-held territory in Syria, where President Bashar al-Assad has taken back most of the ground once held by his enemies with Russian and Iranian support.
The government's latest Idlib advance has triggered a fresh wave of civilian displacement, with hundreds of thousands moving towards the Turkish border. Turkish President Tayyip Erdogan said on Friday Turkey may launch a military operation in Idlib unless the fighting there is halted.
US special envoy for Syria James Jeffrey said on Thursday the Idlib fighting raised the spectre of an international crisis.
Turkey, which already hosts 3.6 million refugees from Syria, fears a fresh wave of migrants from Idlib. It has 12 military observation posts around Idlib, set up under a 2017 agreement with Russia and Iran, and several of them have since been surrounded by advancing Syrian government forces.
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