Four children where among 13 civilians killed in a Russian air strike on a camp for the displaced near the town of Hass in southern Idlib, said the Britain-based Syrian Observatory for Human Rights.

Two other children were killed in regime air raids in different parts of the region controlled by Syria's former Al-Qaeda affiliate Hayat Tahrir al-Sham (HTS), the monitor said.

The strikes came as regime forces battled HTS jihadists and allied rebels in the region, where fierce fighting Friday claimed the lives of 13 loyalists and 18 jihadists and allied rebels, the Observatory said.

Over the past week, pro-regime fighters have advanced on the southern edges of Idlib province, with the aim of capturing the town of Khan Sheikhoun, which 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 regime forces from rebels in December 2016.

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 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.

The violence has 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.