Beirut (AFP) - Al-Qaeda's Syrian affiliate and its allies seized the last regime-held military base in northwestern Idlib province on Wednesday, in the latest setback for President Bashar al-Assad's forces.

Al-Nusra Front and a coalition of mostly Islamist groups captured the Abu Duhur military airport after a siege that lasted two years, the Syrian Observatory for Human Rights monitor said.

"After a violent attack that had been taking place since Monday, Al-Nusra Front and some Islamist factions control all of the Abu Duhur military airport," the Britain-based Observatory said.

The attacking force began to enter the base on Tuesday night.

Observatory chief Rami Abdel Rahman said the forces appeared to have taken advantage of a massive sandstorm that blanketed much of the Middle East and made it difficult for regime aircraft to carry out strikes.

"The army is no longer present in Idlib province," he added, saying that the soldiers who had been defending the base withdrew towards Aleppo province in the east.

"The regime's only presence left in the province is in the Shiite villages of Fuaa and Kafraya," which are besieged by rebels and are being defended by pro-regime militias and Lebanon's Hezbollah, not the army.

Syrian state television effectively acknowledged the loss, saying troops had left the base.

"After fierce fighting at the Abu Duhur airport, the army, which defended the airport with great courage under total siege for more than two years, abandoned their positions," the broadcaster said.

Abu Duhur has long been a target for opposition forces in the northwestern province.

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Rebel forces had seized the entrance to the airport and several positions on its outskirts in late August in an advance that began with suicide bombers on motorbikes.

The airport's capture is the latest success for a coalition of Islamist and jihadist forces, including Al-Nusra, that calls itself the "Army of Conquest".

They first shot to prominence with the swift overrunning of Idlib's provincial capital in March.

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They have since extended their gains throughout the province, seizing a string of strategic towns and even pushing into neighbouring Hama province, where they are fighting to capture the Sahl al-Ghab area.

With Abu Duhur's fall, the regime's only remaining presence in Idlib is in Fuaa and Kafraya.

Both are now completely surrounded by rebel forces, who have fired hundreds of rockets at the Shiite-majority areas.

The loss is the latest setback for Assad's forces, which the embattled president acknowledged in July were suffering from "fatigue".

"There is a lack of human resources" in the army, Assad said in the remarks after a string of defeats in Idlib province and elsewhere.

"The problem facing the military is not related to planning but to fatigue."

Assad's regime has been at war with different rebel groups for the past four years, in a conflict that has so far killed at least 240,000 people.

The conflict began with anti-government protests but descended into a war after a regime crackdown on dissent.

It has evolved into a complex multi-front battle involving rebels, jihadists, the regime, Kurds and a US-led coalition that is carrying out air strikes against the Islamic State group.