Two activist groups, the Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees said the attack late last 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.
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.
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 Organisation 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.
