Over the past week, pro-Assad fighters have advanced on the southern edges of Idlib province, controlled by Syria's former Al-Qaeda affiliate Hayat Tahrir al-Sham (HTS).

On Thursday, regime loyalists stood just three kilometres (1.8 miles) away from the key town of Khan Sheikhun, after capturing five villages to its northwest overnight, said the Syrian Observatory for Human Rights.

The town lies on a key highway coveted by the regime.

The road in question runs through Idlib, connecting government-held Damascus with the northern city of Aleppo, which was retaken by loyalists from rebels in December 2016.

"The aim of the advance is to surround Khan Sheikhun and reach the highway," Observatory head Rami Abdul Rahman told AFP.

Fighting in southern Idlib on Thursday killed five regime combatants and 11 militants and allied rebels, said the Britain-based monitor.

A day earlier militants downed a regime plane near Khan Sheikhun, and took the pilot prisoner.

A video released by HTS on Thursday purported to show the captured pilot, who identified himself as General Mohammad Ahmad Sleiman of the Syrian air force.

HTS has since January controlled most of Idlib province as well as parts of neighbouring Hama, Aleppo and Latakia provinces.

A buffer zone deal brokered by Russia and Turkey last year was supposed to protect the Idlib region's three million inhabitants from an all-out regime offensive, but it was never fully implemented.

Regime and Russian air strikes and shelling since late April have killed more than 820 civilians, according to the Observatory.

It said more than 1,280 militant fighters and 1,140 regime forces have died in the same period.

The violence has also displaced 400,000 people, according to the United Nations.

AFP correspondents have reported seeing dozens of families flee fighting over the past few days, heading north in trucks stacked high with belongings.

Syria's conflict has killed more than 370,000 people and displaced millions at home and abroad since starting with the brutal repression of anti-regime protests in 2011.