Turkish-backed Syrian rebels attacked government-held positions northeast of Aleppo on Saturday, rebel sources and a war monitor said, opening a new front against Syrian army forces that have made significant advances in nearby Idlib over the last week. The attack was focused on territory near the city of al-Bab, which has been controlled by Turkey and its Syrian opposition allies since 2017. Syrian state media made no mention of a new attack. Turkish forces were not taking part, rebel sources said. 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 airpower, have made rapid advances in Idlib this week, capturing the town of Maarat al-Numan which is located about 100 km (60 miles) 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 governments 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. Turkey fears a fresh wave of migrants from Idlib and has 12 observation posts in the region, part of a 2018 de-escalation deal that Erdogan says Russia is now violating. Speaking in Ankara, Erdogan said Ankara will not allow new threats near its borders, even if it meant resorting to military power, as it did in its three previous cross-border operations in northern Syria. US special envoy for Syria James Jeffrey said on Thursday the Idlib fighting raised the specter of an international crisis. Turkey currently hosts more than 3.6 million Syrian refugees. Erdogan has repeatedly called for Assad to step down, even while Iran, Russia, and Turkey have said they seek a political solution to the conflict.