BEIRUT, Lebanon (AP) — A Syrian government airstrike on a town in the country’s northwest killed at least 20 people yesterday, shattering store fronts, setting cars ablaze and sending a giant plume of black and gray smoke into the sky.

President Bashar Assad’s air force has been one of his biggest assets in the two-year-old civil war and he has used warplanes and helicopters to try to check rebel advances, although the regime also frequently hits civilian areas.

A Human Rights Watch report last week accused the Syrian government of committing war crimes by using indiscriminate and sometimes deliberate airstrikes against civilians, killing at least 4,300 people since the summer.

Yesterday’s air raid struck the town of Saraqeb in Idlib province, according to the Britain-based Syrian Observatory for Human Rights activist group. The Observatory said three children were among the 20 people killed in the attack.

Amateur videos posted online showed a giant plume of black smoke, and people in cars and on motorbikes racing to help the wounded. A group of men could be seen carrying a wounded man covered in gray dust. Another man in the video rushes with a bucket of water to help extinguish cars in flames. Rubble and twisted metal litter the street.

The video appeared genuine and corresponded to other reporting by The Associated Press of the events depicted.

Rebels have wrested much of the countryside of Idlib and other provinces in northern Syria from regime forces, although government troops still control many military bases in the region from which they launch attacks — including airstrikes — on opposition-held areas.

South of Saraqeb, Syrian government troops trying to relieve a besieged military base ambushed a rebel checkpoint, killing at least 21 opposition fighters.

The Observatory said the government forces surprised the rebel fighters on the outskirts of the village of Baboulein. The Observatory, which relies on a network of local activists on the ground, said many opposition fighters were also wounded in the attack.

Observatory director Rami Abdul-Rahman said the assault was part of government efforts to resupply the embattled military base at Wadi Deif outside of the town of Maaret al-Numan, which is just north of Baboulein on the Damascus-Aleppo highway.

Rebels have been trying for months to capture the large base at Wadi Deif, from which regime troops regularly pound the now largely abandoned town of Maaret al-Numan with artillery fire. The regime must push convoys through rebel-held territory to prevent the base from running short of troops and supplies.

On Thursday, activists said rebels shot down a helicopter carrying food and supplies to the base, killing the pilot and three other soldiers.

In the northern city of Aleppo, a government air raid on the disputed Sheikh Maqsoud neighbourhood killed at least four people and wounded more than a dozen others, the Observatory said. It added that doctors treating the wounded said many showed symptoms of inhaling toxic gas, such as severe vomiting and irritation to the nose and eyes.