The pullback is the first major test of a deal brokered by government ally Russia and Turkey last month to avoid the humanitarian consequences of a major government offensive, AFP reported.
Under the agreement, all factions have until Wednesday to withdraw heavy weaponry from the 15- to 20-kilometre (nine- to 12-mile) wide buffer zone, which rings Idlib province and adjacent areas of the northwest.
And by Monday, the buffer zone must be free of all terrorists, including those of the region's dominant armed group, the Hayat Tahrir al-Sham (HTS) alliance led by al-Qaeda's former Syria branch.
Analysts had expected Ankara to have a difficult time enforcing the September 17 deal but by Tuesday, the Syrian Observatory for Human Rights said the heavy weapons pullout was nearly complete.
Militants and terrorist factions “have withdrawn their heavy arms from most positions inside the planned buffer zone except for the northern Latakia countryside," the head of the Britain-based monitor said.
"We did not observe any movement of heavy weapons outside that area. They could have been moved to trenches or secret locations," Rami Abdel Rahman said.
The Idlib region consists of a large part of the province of the same name, as well as adjacent militant or terrorist-held areas of the Latakia, Hama and Aleppo provinces.
Tanks and artillery guns
On Tuesday, an AFP correspondent saw tanks and artillery guns that pro-Ankara militants had withdrawn from the buffer zone some 20 kilometers (12 miles) away.
They moved the armored vehicles into pits surrounded by mounds of earth, and wrapped up artillery guns for protection.
HTS and smaller terrorist factions quietly began withdrawing their heavy arms on Saturday in an operation that continued through Monday night, the observatory said.
The pro-Ankara National Liberation Front said it had completed its weapons pullback on Monday.
HTS, which controls more than two-thirds of the buffer zone around Idlib along with other terrorists, has not given any formal response to the September 17 truce deal.
But by beginning to pull out its weapons, the group was "de facto" implementing it, Abdel Rahman said.
"No faction would be able to withstand the consequences of any escalation if the deal's terms were not met," Abdel Rahman said.
Under the terms of the deal, the buffer zone is to be patrolled by Turkish troops and Russian military police.
But terrorists objected to the Russian presence in the zone, and said they received guarantees from Turkey that Russian patrols had been dropped.
For the zone to come into effect, "radical groups" – interpreted as meaning all terrorists must leave the area by next Monday.
It is still unclear whether the terrorists will comply with this second deadline.
Forces loyal to President Bashar al-Assad have retaken swathes of territory in Syria since Russia intervened in September 2015.
A series of offensives this year saw a succession of longtime terror strongholds surrender. A similar Russia-backed assault had been expected against Idlib before the deal was announced last month.
Despite progress in implementing the accord, Assad insisted on Sunday it was a "temporary measure" and Idlib would eventually return to state control.
The Syrian war has killed more than 360,000 people since it erupted in 2011.