The foreign forces are on Syrian soil illegally under the pretext of fighting terrorism and "will be dealt with accordingly", Syrian Foreign Minister and Deputy Prime Minister Walid al-Moualem told the UN General Assembly on Saturday.
"They must withdraw immediately and without any conditions." he said.
France has more than 1,000 troops there and the US has about 2,000, mainly training and advising Kurdish and Syrian Arab forces opposed to Syrian President Bashar al-Assad.
Moualem also called on refugees to return, arguing that "some western countries" are "spreading irrational fears" to discourage them from going home.
Proposed buffer zone
Fighters from Faylaq al-Sham, which is part of the Turkish-backed alliance National Liberation Front (NLF), started withdrawing from the proposed buffer zone on Sunday morning.
The group has some 8,500 to 10,000 fighters.
But most of the 15-20km-wide area is controlled by either hardline jihadists and Hayat Tahrir al-Sham, which is led by former members of Al-Qaeda's Syria branch and widely considered the most powerful force in Idlib.
Two groups, Hurras al-Deen and Jaish al-Izza have rejected the proposal, which was agreed by Russian President Vladimir Putin and Turkish counterpart Recep Tayyip Erdogan on 17 September.