The Syrian Observatory for Human Rights said the Faylaq Al Sham rebel group has began its withdrawal from parts of the neighbouring province of Aleppo on Sunday "with heavy weapons, including tanks and cannons".
It said the buffer zone should be carved out equally from both rebel-held territory and nearby zones controlled by forces loyal to Syrian President Bashar al-Assad.
Pro-Turkey rebels have cautiously accepted the deal, but the Jaysh al-Izza faction on Saturday rejected it, saying the zone to be set up by October 15 would only encompass territory now under rebel control.
Under the accord reached on September 17, rebel and opposition factions would withdraw their heavy weaponry from the buffer zone. "This report is denied, completely denied", NLF spokesman Naji Mustafa told AFP.
Faylaq al-Rahman includes between 8,500 and 10,000 fighters and is one of the factions of NLF, which was established in August with the support of Ankara in Idlib province and neighboring areas that fall under the control of fighting factions in Aleppo, Hama and Lattakia, he explained.
The sources said that the foreign terrorists are still stationed in the villages, towns and occupied positions, adding that the foreign terrorist groups, including Turkistanti party, Ajnad al-Qoqaz and Horas al-Din, have strengthened their military preparedness to fight against the Syrian army.
It is aimed at countering the growing power of Hay'at Tahrir al-Sham (HTS), led by al-Qaeda's former Syrian affiliate, which controls 60 percent of Idlib.
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Al-Qaeda loyalists Hurras al-Deen, which have a presence in the zone, rejected the deal last week.
The rest is held by the NLF and other rebels.
The London-based observatory also reported clashes between militants and government forces in the coastal province of Latakia on Sunday.
The group also said Turkey should ensure the Idlib deal does not end up like last year's de-escalation zones in central and southern Syria, where rebel-held areas were later simply taken over by government forces.
Idlib and adjacent militant territory are home to some three million people, about half of them displaced from other parts of Syria.
The areas still outside his control are Idlib in the northwest, and a northeastern chunk held by Kurdish authorities where USA and other Western troops are present.
The foreign ministers met on the sidelines of the United Nations General Assembly, Russian news agency Tass said.