Syrian President Bashar al-Assad on Monday vowed his forces would be relentless in their offensive to flush out the last rebel bastion in northern Syria. His address on state television came even as the United Nations' chief relief coordinator Mark Lowcock renewed the world body's appeal for a ceasefire in the Idlib region.

"The battle for the liberation of the provinces of Aleppo and Idlib continue, regardless of all the hot air coming out of the north," Assad said, in reference to warnings by Turkey. In recent weeks, Syrian troops and allied forces backed by Russia have stepped up their offensive against rebel and jihadist groups in Idlib and the neighbouring province of Aleppo.

They have already reconquered swathes of Idlib as well as key areas that have secured the crucial M5 highway connecting the country's four largest cities as well as the entire surroundings of Aleppo city for the first time since 2012.

"We have won a victory over the fear they tried to instill in our hearts... but we are fully aware that this liberation is not the end of the war," Assad said. "But this liberation definitely does mean we have rubbed their faces in the dirt, as a prelude to their total defeat, which will come sooner or later," he said.

Deadly incidents between regime forces and rebel-backer Turkey have raised tensions near Syria's northern border, prompting Turkish President Recep Tayyip Erdogan to threaten Damascus. Aid groups have called for a cessation of hostilities in northwestern Syria, where around three million people are trapped in the rebels' ever-shrinking enclave.

With Turkey unwilling to open its borders to another wave of refugees and government troops closing in from several directions, the UN has warned of a humanitarian disaster in proportions yet unseen since the start of the war in 2011. The UN's top humanitarian coordinator, Mark Lowcock said in a statement on Monday that 900,000 people had been displaced since December.