At least 18 people, including women and children, were killed in airstrikes on areas in the last major rebel stronghold in northwest Syria on Saturday as a three-month truce crumbles.

The Syrian Observatory for Human Rights said 20 people were killed in Idlib province while the opposition's Syrian Civil Defense said 18 lost their lives.

The airstrikes on Idlib province have intensified over the past few weeks as the government appears to be preparing for an offensive on rebel-held areas east of the province to secure the main highway that links Damascus with the northern city of Aleppo.

A fragile cease-fire halted the government advance in late August but has been repeatedly violated in recent weeks.

Conflicting casualty figures are common in the immediate aftermath of violence in Syria, where an eight-year conflict has killed about 400,000 people, wounded more than a million and displaced half the country's prewar population.