The main Western-backed Syrian opposition group is calling on the United Nations to send a fact-finding mission to investigate an alleged poison gas attack on a rebel-held town.

The Syrian National Coalition and activists inside Syria say the government carried out a chlorine gas attack on the town of Sarmin in northwestern Syria late Monday, killing six people and leaving dozens more struggling to breathe. Syrian authorities deny the allegations.

Hisham Marwa, vice president of the Syrian National Coalition, called for an on-site UN investigation as soon as possible.

In a statement released late Tuesday, he also demanded that the Security Council enforce a recent resolution condemning the use of toxic chemicals such as chlorine in Syria. That measure also threatens military action in case of further violations.