A Syrian government airstrike on a town in the country's northwest killed at least 20 people Saturday, shattering store fronts and setting cars ablaze.

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 this 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.

Saturday'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 grey 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.

Military bases

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.

Helicopter downed

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.

Both sides in the Syrian civil war have accused the other of using chemical weapons.

Syria has asked the UN to investigate an alleged chemical weapons attack by rebels in March on the village of Khan al-Assal outside of Aleppo. The rebels blame regime forces.

Britain and France want the UN to investigate allegations of chemical weapons use in Khan al-Assal and another village, Ataybah, on March 19, as well as the central city of Homs on Dec. 23.

Syria has rejected UN Secretary-General Ban Ki-moon's push to expand the probe to include those other villages.

The Syrian conflict began in March 2011 with largely peaceful protests but has since devolved into a civil war that the United Nations says has killed at least 70,000 people. Hundreds of thousands of Syrians have sought refuge abroad, and millions more have fled their homes to try to find safety elsewhere inside Syria.

International efforts to find a peaceful resolution to the conflict have faltered.