A deadline for jihadists to leave a planned buffer zone around Syria's last major rebel bastion of Idlib passed Monday without any sign they had done so, said the Syrian Observatory for Human Rights.

A planned buffer zone in northwest Syria has been cleared of heavy armaments ahead of time but a new deadline loomed Wednesday for the tougher task of Turkey convincing jihadists to pull out their fighters.

Jihadists and Turkish-backed rebels in Syria's last major opposition stronghold have withdrawn most of their heavy weapons from a planned buffer zone ahead of a Wednesday deadline, a monitor said.

The pullback is the first major test of a deal brokered by government ally Russia and rebel-backer Turkey last month to avoid what the United Nations warned would be the appalling humanitarian consequences of a major government offensive.

A Turkish military convoy entered rebel-held northwestern Syria early on Wednesday, an AFP correspondent reported, as the deadline for establishing a buffer zone between rebel and government forces draws closer.

Russia on Friday warned it will not allow jihadists in Syria to be sent to Afghanistan or elsewhere under a deal reached with Turkey that averted a large-scale military assault on rebel-held Idlib province.

Turkish troop reinforcements entered Syria's rebel bastion of Idlib on Tuesday, an AFP correspondent reported, a week after a deal between Ankara and Moscow averted a government offensive.

Around 35 military vehicles travelled south down the main highway near the town of Saraqib after midnight.

Russia's Vladimir Putin on Monday spoke to Syria's Bashar al-Assad for the first time since last week's downing of a Russian military plane, telling him of Moscow's plan to deliver a new S-300 air defense system to the Syrian army.

Israel's coordination with Russia on its attacks in Syria remains unchanged despite the deadly downing of a Russian plane, and the Jewish state retains its right to decide on military actions, an Israeli official said.

The deal to avoid a Syrian government offensive on Idlib province is still being worked out by Russia and Turkey, the UN said Thursday, stressing that the threat to civilians remained high.

Iran's foreign minister on Tuesday hailed an agreement between Turkey and Russia to avert an assault on Syria's last major rebel stronghold in Idlib province as an example of "responsible diplomacy."