The purported use of poison gas on the town of Sarmin in northwestern Idlib province is the first alleged chemical attack since the UN Security Council approved a US-drafted resolution this month that condemns the use of toxic chemicals such as chlorine in Syria.
That measure also threatens military action in case of further violations.
But any such action would require the consent of the Security Council, which remains deeply divided over Syria's civil war.
The attack on Sarmin, located some eight kilometres (five miles) east of Idlib city, took place late Monday, according to the Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees activist collective.
Also Read
The two activist groups said yesterday that six people were killed and dozens more suffered from severe breathing difficulties.
A Syrian military official in Damascus denied any government role in the attack and blamed it on rebels.
"The army did not and will never use any internationally-prohibited weapon," the official said on condition of anonymity because he was not authorized to brief the media.The main Western-backed opposition group, the Syrian National Coalition, said government helicopters dropped four "barrel bombs," two of which contained chlorine gas. It said about 70 people suffered breathing problems.
In a letter to the Security Council president, the coalition's representative to the UN, Najib Ghadbian, said individual member states should bypass the deadlocked council and establish a no-fly zone over part of Syria to protect civilians.