Saraqib (Syria) (AFP) - Turkey beefed up its military positions in Syria's rebel bastion of Idlib Tuesday as the clock ticked down on a mid-October deadline to remove jihadists from the area.

Further complicating Turkey's task, a war monitor said hundreds of members of the Islamic State group were transferred by the regime to the northwestern region from eastern Syria.

Russia, the government's main ally, and Turkey, the rebels' top sponsor, agreed last week in Sochi on a plan meant to avert a large-scale regime offensive on Idlib.

The country's last major rebel stronghold is home to around three million people, and the United Nations had warned an assault could have sparked a humanitarian disaster on a scale yet unseen in the seven-year conflict.

The deal reached in the Russian resort puts the onus on Turkey, which is now expected to get jihadists to hand over their heavy weapons and vacate a U-shaped demilitarised zone around Idlib.

Turkey already has 12 military "observation points" dotted across the province, and on Tuesday an AFP correspondent saw a convoy of reinforcements after they crossed the border into Idlib.

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

The convoy was accompanied by pro-Ankara rebels of the National Liberation Front (NLF), who control part of the enclave on the Turkish border.

- IS fighters arrive -

Hayat Tahrir al-Sham (HTS), a jihadist alliance led by Syria's former Al-Qaeda affiliate, controls more than half of the rebel zone, while NLF fighters hold sway over most of the rest.

The Syrian Observatory for Human Rights said the Turkish convoy headed to an "observation point" in the north of Hama province.

On Monday, the Observatory said the government had transferred hundreds of rival IS jihadists to the area.

The head of the Britain-based monitor, Rami Abdel Rahman, said they were brought from an area where IS still holds a few pockets near the Iraqi border in Deir Ezzor province.

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"Regime forces transported more than 400 IS fighters late Sunday from the desert near the town of Albu Kamal," he said.

They arrived in the eastern countryside of Idlib at dawn on Monday, near areas where other jihadist groups are present, he said.

Abdel Rahman said the transfer came after days of negotiations and as the Syrian army with allied Iranian forces threatened an operation against IS.

"An agreement was reached with IS fighters in one of their pockets in the desert south of Albu Kamal to evacuate them," he said.

- Excuse for an assault -

Observers have already said Turkey's task was almost impossible and that the presence of IS, over which it has very limited sway, would further muddy the waters.

Tore Hamming, a Copenhagen-based academic specialised in the study of jihadist groups, said the IS transfer could spark clashes between rival factions inside Idlib.

Beyond exacerbating differences among its foes, he said the regime may have sought to create a justification for a full-blown Russian-backed offensive.

"It is easier to convince international players to accept an offensive if you can say there are substantial numbers of IS fighters," Hamming said.

The conflict in Syria has killed more than 360,000 people since 2011.

President Bashar al-Assad's regime has ousted opposition fighters and jihadists from large parts of Syria since Russia intervened militarily by its side in 2015.

After regime troops started massing troops around Idlib last month, the United Nations sounded the alarm over a possible "bloodbath".

Following several accusations that the Syrian regime has used chemical weapons during the conflict, the United States warned Damascus that any use of toxic arms would not be tolerated.

On Tuesday, as the UN Assembly opened in New York, US President Donald Trump vowed a response to any chemical attack in Syria.