Sunni Islamist groups have overrun Syrian army outposts and villages in the Idlib province, closing on coastal strongholds of President Bashar al Assad's government, rebels and a monitor said on Saturday.

The Syrian army said its troops had abandoned the town of Muhambal in the western part of Idlib and were regrouping for a counter-offensive. Social media videos showed army trucks and large caches of weaponry abandoned.

The Islamist alliance, including al Qaeda's Nusra Front, calls its operation the "Army of Fatah", a reference to conquests that spread Islam across the Middle East from the seventh century.

"After taking over several villages, we came from the mountains and entered the town and began combing it," Abu Malek, a field commander from Nusra who led the advance against Muhambal, was quoted as saying on an opposition television station.

Muhambal lies close to a highway that runs between Aleppo in the north and the port city of Latakia.

Syrian state television later said it killed "tens of terrorists" and destroyed large convoys carrying Nusra Front fighters in a series of air raids on their positions in Basankoul and a string of villages rebels had taken over.

Heavy fighting also broke out around Abu Al Dahour military airport, the largest Syrian airport in the north of the country, and it's last stronghold in the eastern part of Idlib province.

The army said it had killed scores of jihadists holed in surrounding villages east and north of the major military installation besieged unsuccessfully by Islamist rebels for over a year but who have now renewed their offensive.

The Syrian Observatory for Human Rights, which tracks violence across the country, said large caches of weapons were abandoned by the army when they fled the area.

Rebels have used American manufactured TOW anti-tank missiles against the Syrian army in weeks of heavy fighting, weapons that Damascus accuses Turkey and Saudi Arabia of supplying to insurgents seeking to topple Assad's government.

A video released by a rebel group showed a Syrian tank on fire after being directly hit by what it claimed was a TOW missile near a bridge close to the town of Mohambal.

The main olive growing Idlib province is strategically located, bordering Turkey and adjoining Latakia, the coastal province on the Mediterranean whose mountains are the ancestral home of President Assad's minority Alawite sect.

The British-based Observatory said the Islamist insurgents intensified mortar shelling on army outposts in the strategic Jabal al Akrad mountain range that overlooks Alawite villages and close to Qardaha, hometown of the Assad family.

Nusra Front are rivals of the ultra hardline Sunni militants of Islamic State who also expanded their presence after taking control of the central city of Palmyra last month. It marked the first time the group had seized a Syrian city directly from government control.

Along the Syrian-Lebanese border, the Syrian army and its ally the Lebanese Hezbollah group announced on Saturday they had made new strategic gains in their month-long campaign to flush out Nusra Front's supply lines along the porous border.-Reuters