Regime ally Russia and rebel backer Turkey agreed last month to set up a demilitarised zone around the northwestern region of Idlib to prevent a bloody regime assault on the region.
Under the deal the horseshoe-shaped area was to be free of heavy arms by October 10 with "radical fighters" pulling out by Monday.
While the deadline for withdrawing heavy weapons was met on time, there has been no indication that the second condition is being implemented.
"We have not monitored any withdrawals by jihadist fighters at all from areas falling in the planned buffer zone," said the Britain-based Syrian Observatory for Human Rights monitoring group.
An AFP correspondent in Idlib also said no hardline factions had moved any of their units in recent days.
Idlib and surrounding rebel zones are held by a complex array of factions.
Less than half is controlled by the Ankara-backed National Liberation Front (NLF), the main rebel conglomerate there.
But the lion's share is held by Hayat Tahrir al-Sham (HTS), an alliance led by former Al-Qaeda members, as well as more hardline jihadists like Hurras al-Deen and Ansar al-Islam.
Those fighters also control more than two-thirds of the planned buffer zone and are supposed to leave it by Monday.
Hurras al-Deen has publicly rejected the agreement.
HTS, widely considered the most powerful force in the area, has not publicly commented on the accord but quietly abided by its first deadline and re-stationed its heavy arms elsewhere.
Observers have said that getting it to execute the deal's second half would be much more challenging.
In a recent report for the Turkey-based Omran Center, expert Nawar Oliver described HTS's approval as the deal's ultimate "test".
"If HTS acts as a spoiler to the agreement on the ground, this will probably lead to one of two scenarios: either Turkey and the NLF launch military action against HTS, or Russia will seize the opportunity with the support of the regime and its allies to enter Idlib," he said.
"The ramifications of that move could be vast," he added.