Beirut - Militants in Syria indicated tepid support for a demilitarised zone in the country’s final opposition stronghold, even as they appeared to defy an internationally-brokered deadline for their withdrawal.

The area surrounding Syria’s northern province of Idlib is home to some 2.5 million people, most of them civilians. Aid groups have warned that an offensive there could spell humanitarian catastrophe.

A September 17 deal between Russia and Turkey - key backers of Syria’s regime and rebel forces, respectively - was meant to have seen heavy weapons and militant groups pulled out of a roughly 15-20km wide buffer zone.

But as the deadline passed, monitoring groups said that the militants were still there.

“We value the efforts of all those striving - at home and abroad - to protect the liberated area and prevent its invasion and the perpetration of massacres in it,” said the Al Qaida-linked Hayat Tahrir Al Sham, in a statement late Sunday.

“But we warn at the same time against the trickery of the Russian occupier or having faith in its intentions,” it added. The group also said it “would not forget” the foreign fighters who came to assist it.

Russian officials emphasized last week that deadline for implementation was viewed as flexible, indicating that the militants’ failure to withdraw would not meet with immediate violence. “The quality of that work is far more important. We strongly support our Turkish partners’ efforts,” said Russian Foreign Minister Sergei Lavrov on Wednesday.