Ahmad Jarba, leader of the Syrian National Coalition, called on the United States, Britain, France, as well as Arab states, to take "serious steps and actions" after the attack in a Damascus suburb on Wednesday.
"I ask the international community to move from words into actions, we've had enough with words and we need serious steps and actions," he said at a press conference in Istanbul.
US President Barack Obama should lead an intervention along with the leaders of Britain and France, he said.
The leader of the coalition described the response so far as a "shame" for not reaching "the ethical and legal response that Syrians expect".
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US President Barack Obama meanwhile met his top national security advisers to weigh the response to the killings.
He is under mounting pressure to act following Wednesday's reported chemical attack near Damascus that opposition groups say was carried out by President Bashar al-Assad's forces and claimed up to 1,300 lives.
The Syrian government has strongly denied the allegations but has yet to accede to demands that UN inspectors already in the country be allowed to visit the sites of the alleged attacks.