GENEVA (AP) — The U.N.’s special envoy to Syria Lakhdar Brahimi says evidence suggests that some kind of chemical “substance” was used in an attack that killed hundreds of people, but said any military strike on Syria must have U.N. Security Council approval.

Brahimi spoke to reporters Wednesday in Geneva as a U.N. inspection team was investigating the alleged poison gas attack near Damascus on Aug. 21 and momentum built for Western military action against Syrian President Bashar Assad’s.

Brahimi said that “it does seem like some kind of substance was used” that killed hundreds of people, but “international law says that any U.S.-led military action must be taken after” agreement in the 15-nation Security Council.

He added that President Barack Obama’s administration is “not known to be trigger-happy.”