As part of the agreement brokered last month, “radical” fighters were supposed to withdraw from frontlines by midnight on Sunday, allowing for the creation of a demilitarised zone that would be patrolled by Russian and Turkish forces.
But that deadline passed without any major movement of fighters from the proposed buffer zone, according to the Syrian Observatory for Human Rights, a UK-based monitor.
The development threatens to undermine a deal that was widely credited with averting a humanitarian disaster for the estimated three million civilians of Idlib.
Syria's foreign minister, Walid al-Moualem, said that he would wait for Russia’s response, but again raised the prospect of an offensive.
"Our armed forces are ready around Idlib to eradicate terrorism if the Idlib agreement is not implemented," he said Monday.
"Idlib, as any other province, has to return to Syrian sovereignty. We prefer to have it through peaceful means, through reconciliation, but if not there are other options."
The Idlib agreement followed a major build up of Syrian government forces around the borders of the province, and weeks of threats from Damascus to recapture the last remaining rebel-held territory in the country.
Turkey, which backs a number of rebel groups in Idlib, feared an offensive would send another wave of refugees to its borders, while the United Nations warned that such an assault could have caused the “worst humanitarian catastrophe” of the 21st century.
Intensive diplomacy eventually postponed an attack at the last minute. Since then, rebels had shown signs of adhering to a key part of the agreement by removing heavy weaponry from the frontlines.
But an ambiguous statement released on Sunday evening by the extremist Hayat Tahrir al-Sham (HTS) rebel group, a former al-Qaeda affiliate that controls an estimated 60 per cent of the province, has muddied the waters.
While not explicitly backing the deal, the group signalled it would abide by its terms to avoid a government offensive.
"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," Tahrir al-Sham said in its statement, adding that it would not hand over its weapons.
The extremist group is seen by both Turkey and Russia as a potential spoiler of the deal, and is unlikely to be included in any reconciliation agreements with the government.
Charles Lister, a senior fellow at the Middle East Institute, said ambiguity from HTS reflected growing internal divisions over how to respond to the new reality in Idlib.
“Its leadership knows that having a grudging acceptance of the deal on record is the best possible line of defense for now,” Mr Lister told The Independent.
Turkey’s deep involvement in Idlib and moves towards de-ecalastion “have posed existential challenges to a group that has gone from being outwardly pro-al-Qaeda global jihad, to a movement that engages in limited foreign diplomacy and engages in agreements that involve traditional enemies, like Russia and Iran,” he added. “That’s sparked a crisis of identity.”
Nicolas Heras, a fellow at the Center for a New American Security, said the group’s intransigence is likely to raise tensions with Turkey.
“The question is whether Turkey is willing to apply its military power to force the compliance of HTS, especially because Turkey's Syrian rebel proxies in Idlib are not up to the job themselves,” he said.
“The Turks for their part have shown no action to remove any of the jihadist groups from either Idlib or the demilitarized zone, which rightfully calls into question whether Turkey is willing remove the al-Qaeda safe haven in Syria.”
The success of the deal will depend in large part on the flexibility of Russia, whose foreign minister Sergei Lavrov suggested last week that Monday was a soft deadline for withdrawal.
But while its future is uncertain, residents said the agreement has provided some respite from the intermittent bombing and instability that has been a part of daily life in Idlib for some time.
“Since the agreement was announced, people are more relaxed,” said Wissam Zarqa, speaking by phone from the northern countryside of Idlib.
“No one is optimistic that it will last forever. But it’s like a break, and people needed a break,” he added.
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