Syrian activists and opposition members claimed on Tuesday that government helicopters carried out a chlorine gas attack on a northern town overnight, killing six people, an accusation that was promptly dismissed by a military official.

Two activist groups — the Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees — said the attack late on Monday night targeted the town of Sarmin in northwestern Idlib province.

They said that apart from killing six, dozens of people suffered from breathing difficulties in the gas attack. The two groups collect their data from a network of activists on the ground.

A military official in the capital, Damascus, denied the claim and blamed rebels fighting to topple President Bashar Assad for the attack.

An opposition official in the area of Sarmin said there were two attacks, the first targeting rebels that injured 20 people, mostly men, while the second hit a residential area. He said Waref Mohammed Taleb, his wife, mother and three little children were the six people killed.

The military and the opposition official both spoke on condition of anonymity because they were not authorized to talk to reporters.

The main Western-backed opposition group, the Syrian National Coalition, said that in the Sarmin attack, helicopter gunships dropped four "barrel bombs," of which two had chlorine gas. The coalition and the opposition official said about 70 people suffered breathing problems.

Amateur videos posted online and claiming to be from the attack show three children lying on hospital beds as medics try to assist them. The footage shows an apparently dazed child slowly moving his head while lying on a hospital bed. The lifeless body of a woman lies on another bed.

The video, which could not be independently confirmed, appeared genuine and corresponded to other reports of the events depicted.

The attack came nearly two weeks after the U.N. Security Council approved a United States-drafted resolution that condemns the use of toxic chemicals such as chlorine in Syria, while threatening militarily enforced action in case of further violations.

The resolution followed last month's condemnation by the world's chemical weapons watchdog of the use of chlorine in Syria as a breach of international law. The Organization for the Prohibition of Chemical Weapons' fact-finding mission concluded "with a high degree of confidence" that chlorine was used on three villages in Syria last year, killing 13 people.

The resolution threatens action against further violations under a council resolution in 2013 that banned Syria's use of chemical weapons. It applies to any party in the Syrian conflict, now in its fifth year. The civil war has so far killed an estimated 220,000 people, according to U.N. figures.

Earlier Tuesday, a leading international rights group criticized Syrian government bombings last November that targeted the de-facto capital of the Islamic State group, saying the airstrikes killed dozens of civilians and may amount to war crimes.

Amnesty International said in a statement that it has documented a series of Syrian government airstrikes between Nov. 11 and Nov. 29 that killed up to 115 civilians, including 14 children, in the city of Raqqa in northern Syria. On Nov. 25, the AP reported that at least 60 people were killed in airstrikes that day in Raqqa.

Raqqa has been the seat of the Islamic State group since it declared a caliphate in areas under its control in Iraq and Syria.