Evidence suggests that some kind of “substance” was used in Syria that may have killed more than 1,000 people, but any military strike in response must first gain U.N. Security Council approval, special envoy to Syria Lakhdar Brahimi said Wednesday.

Brahimi spoke to reporters in Geneva as U.N. chemical weapons experts headed to a Damascus suburb for another look at the alleged poison gas attack near Damascus on Aug. 21 and as momentum built for Western military action against Syrian President Bashar Assad’s regime.

“With what has happened on the 21st of August last week, it does seem that some kind of substance was used that killed a lot of people: hundreds, definitely more than a hundred, some people say 300, some people say 600, maybe 1,000, maybe more than 1,000 people,” said Brahimi, who has been the U.N. and Arab League’s special envoy to Syria since August 2012.

“This was of course unacceptable. This is outrageous. This confirms how dangerous the situation in Syria is and how important for the Syrians and the international community to really develop the political will to address this issue seriously, and look for a solution for it,” he said.

Brahimi did not say on what he based his information, but he did discuss the work of the U.N. team probing for evidence of the use of chemical weapons.

“The United Nations has inspectors on the spot. They have already spent one day in one area where this substance — whatever it is — has been used,” he said. “They have come back with a lot of samplings, they talked also to doctors, they talked to witnesses.”

He said his information did not come from Western intelligence, including what U.S. Secretary of State John Kerry has called “undeniable” evidence of a large-scale chemical attack likely launched by Assad’s regime.

“What we have been told is that this evidence that the Americans, the British, the French say they have is going to be shared with us. It hasn’t been until now,” he said. “And we will be very, very, very interested in hearing from them what this evidence they have is.”

Syria has one of the world’s largest stockpiles of chemical charges, but denies the charges. U.S. leaders have not presented their proof, and U.N. inspectors have yet to comment on the allegations.

Brahimi called the civil war — which has killed more than 100,000, forced about 2 million refugees from their homes and displaced millions of others — the most serious crisis facing the international community.

But he said that any U.S.-led military action must first gain approval from the 15-nation Security Council, whose five permanent members — Britain, China, France, Russia and the United States — each have veto power. The United States has been laying the groundwork for a possible strike on Syria, but U.N. Secretary-General Ban Ki-moon has urged nations to allow more time for diplomacy.

Britain says it will seek a measure “authorizing necessary measures to protect civilians” in Syria. They want it drafted under Chapter 7 of the U.N. Charter, which is militarily enforceable, when it is a political resolution that doesn’t mandate any specific actions.

“International law says that military action must be taken after a decision by the Security Council,” Brahimi said. “I must say that I do know that President Obama and the American administration are not known to be trigger-happy.-What they will decide, I don’t know. But certainly international law is very clear: the Security Council has to be brought in.”

(AP)