At least three children and 18 others have been killed in a series of airstrikes on a marketplace in Syria.

According to the activist-run Kafranbel Media Centre, the civilians were killed in the opposition-controlled town of Kafranbel in north-western Syria.

The Britain-based Syrian Observatory for Human Rights monitoring group confirmed the deaths, and said members of the local police force were among the group targeted.

Russia's military announced last month it was resuming its airstrikes on the north-western Idlib province and central Homs province in support of the government's efforts to suppress a years-long uprising against its national authority.

However Kafranbel is not on any front lines of the war.

A building in al-Firdous, Aleppo, was badly damaged. Credit: Reuters

Earlier Syria's army offered rebel fighters in besieged eastern Aleppo an opportunity to leave the city with their lives.

Army spokesman Brigadier Gen Samir Suleiman said those who choose to stay will face "inevitable death".

Government forces and allied militias are attempting to collapse an opposition pocket on the city's eastern side.

"We will continue fighting until we restore stability and security to all neighbourhoods" of Aleppo, the brigadier said.

He said the Syrian army had retaken more than 50% of the districts in eastern Aleppo.