The Secretary General of NATO said Monday he wass “convinced” chemical weapons were used by the Syrian forces in an attack last month that left hundreds dead, and called for a “firm” international response.

Speaking at a news conference, Anders Fogh Rasmussen said NATO had received overwhelming evidence pointing to the fact that Syrian soldiers loyal to president Bashar Assad had carried out the deadly assault on August 21.

“I have been presented with concrete information and, without going into details, I can tell you that personally I am convinced, not only that a chemical attack has taken place… but I am also convinced that the Syrian regime is responsible,” Rasmussen said.

The NATO chief said a strong international reaction to the attack was needed in order to show dictators around the world that such weapons cannot be used with impunity.

“We need a firm international response in order to avoid that chemical attacks take place in the future. It would send, I would say, a dangerous signal to dictators all over the world if we stand idly by and don’t react,” he said.

His comment came two days after US President Barack Obama backed off imminent military action against Syria, instead opting to await Congressional approval.

Rasmussen said the alliance would not take part in any military action, but maintained that it would remain a strong defender of Turkey in case the member state was attacked as part of the Syria crisis, and NATO would remain a forum for allies to consult about action.

“If a response to what has happened in Syria were to be a military operation, I’d envisage a very short, measured, targeted operation, and you don’t need the NATO command and control system to conduct such a short, measured, tailored, military operation,” he said.

Earlier this year NATO deployed several Patriot anti-missile batteries to the Turkish Syrian border at Ankara’s request.

Rasmussen’s comments joined a growing chorus of international leaders who have blamed Assad for the attack. On Sunday, US Secretary of State John Kerry said the chemical used in the attack, which Washington says killed over 1,400 people, had signatures of sarin gas.

He said that a “case is building” for a military attack.

“This case is going to build stronger and stronger,” he said on NBC’s “Meet the Press.” But he also said he thinks “the people of America should be celebrating that the president is not acting unilaterally.”

Britain recently pulled back from joining a military response, after Parliament nixed the idea, handing Prime Minister David Cameron a stinging defeat. French President Francois Hollande has said it will also wait for lawmakers’ approval before considering military action.