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Air strikes Friday by Bashar al-Assad's brutal regime and its Russian ally killed 15 civilians, most of them in a camp for the displaced in Idlib province, a war monitor said. 

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 forces and allied rebels in the region, where fierce fighting Friday claimed the lives of 13 Assad loyalists and 18 HTS fighters 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 Syrian war began when the Baath regime, in power since 1963 and led by President Bashar al-Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.

At least tens of thousands of civilians have been killed in the war, mostly by the regime and its powerful allies, and millions have been displaced both inside and outside of Syria.

The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.

Agencies contributed to this report.