NATO Secretary General Anders Fogh Rasmussen said on Monday he had seen evidence that had convinced him that Syria was responsible for a chemical weapons attack 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," he said at a press conference in Brussels.
"We believe that these unspeakable actions which claimed the lives of hundreds of men, women and children cannot be ignored," he added.
"I think there is an agreement that 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.
French Prime Minister Jean-Marc Ayrault was due to present France's evidence that chemical weapons were used in Syria at a meeting Monday with leaderes of the Parliament on Monday.
The dossier is said to show that Syria has stockpiled more than 1,000 tonnes of chemical agents, including sarin.
French President Francois Hollande, like US President Barack Obama, is in favour of a military action against Syria.
But there is growing pressure for a vote in the parliament on the issue, to be in line with the US and also Britain.
French MPs are due to debate the issue on Wednesday while, at US President Barack Obama’s request whos is seeking Congressional support, American lawmakers are due to reconvene next week.
British Prime Minister David Cameron last week suffered a painful defeat in arliament when 285 MPs voted No to his resolution authorising force to protect Syrian civilians against 272 who voted Yes.
The decision however did not rule our future British military intervention.
French President Francois Hollande has said he is ready to join the US in strikes on Syria after British MPs voted No.
Asked in an interview in the French daily Le Monde on Friday, the day after the UK decision, if he would take part in an attack without British help, he said: “Yes."
He added: "Each country is sovereign in deciding whether or not to participate in an operation. That’s true for the UK as it is for France. I’ll be having a serious talk today with Barack Obama.”
He noted: "There are few countries that have the capacity to inflict a sanction by the appropriate means. France is one of them. We are ready. We will decide our position in close liaison with our allies."
He also said: "All the options are on the table. France wants action that is proportional and firm against the Damascus regime."