Human Rights Watch (HRW) sharply criticised the Syrian air force Wednesday for intensifying strikes on Aleppo, despite a UN Security Council resolution ordering all sides in the conflict to stop indiscriminate attacks.

The New York-based group also rapped the Security Council for inaction over the violence in Syria in a statement issued ahead of a meeting of the UN body.

"The Syrian government is raining high explosive barrel bombs on civilians in defiance of a unanimous United Nations Security Council resolution," HRW said, referring to resolution 2139 from February.

The resolution banned the indiscriminate use of barrel bombs -- widely used by Syrian government forces on rebel-held towns and cities -- and all other weapons in populated areas.

"Month after month, the Security Council has sat idly by as the government defied its demands with new barrel bomb attacks on Syrian civilians," HRW's Sarah Leah Whitson said.

The group also urged two key backers of President Bashar al-Assad, Russia and China, to allow the Security Council to implement the resolution.

HRW said it had documented "over 650 major new damage sites consistent with barrel bomb impacts" in opposition-held areas of Aleppo city since the resolution was passed.

"Witness statements, satellite imagery analysis, and video and photographic evidence obtained by Human Rights Watch indicate that government forces have maintained and even increased their bombardment rate of Aleppo," the group said.

HRW described barrel bombs as "cheaply made, locally produced, and typically constructed from large oil drums, gas cylinders, and water tanks, filled with high explosives and scrap metal to enhance fragmentation, and then dropped from helicopters".

It criticised rebel forces for carrying out "indiscriminate attacks as well, including car bombings and mortar attacks in pro-government areas".

The group added: "The deliberate targeting of civilians is a war crime, and if carried out in a widespread or systematic way as part of a policy of the government or an organised group, can amount to crimes against humanity".

According to the Syrian Observatory for Human Rights, hundreds of civilians, including children, have been killed in a major aerial offensive launched by government forces on Aleppo in December.

On Wednesday alone, five civilians were killed in a barrel bomb attack on the Salheen neighbourhood of eastern Aleppo city, it said.

"Another eight people are believed to be under the rubble, but we do not know whether they are dead or alive," said Observatory director Rami Abdel Rahman.

Twelve people were killed, among them a child, in Syrian army shelling late Wednesday on Douma, a rebel-held town near Damascus, the monitoring group added.

On Tuesday, six civilians were killed, among them a child and an elderly man, in an air strike on the Bab al-Nairab neighbourhood, the Observatory said.

Regime-held areas also suffered attacks on Wednesday.

In the central city of Homs, two people were killed and 18 others wounded in a car bomb attack on the majority Alawite district of Wadi al-Dahab, according to the Observatory.

State television had earlier reported a death toll of one, and had said the attack had been carried out by a suicide bomber.

Three people were killed, meanwhile, in mortar attacks on regime-held areas of Aleppo city, said the Observatory.

The war in Syria has killed more than 170,000 people, and forced nearly half the population to flee.