At last, the eyewitness testimony U.N. investigators have been waiting for.

Amateur video appears to show a victim of last week's apparent poison gas attack outside Damascus speaking with the U.N. chemical weapons team that's come to investigate.

SOUNDBITE (Arabic) UNIDENTIFIED MALE VICTIM, SAYING

"There are chemicals, and then we went outside to bring ambulances, (translator asks, what did you feel?) I started to get dizzy, and I couldn't see anymore, and I wanted to throw up. I didn't see in front of me and then I fell down. The situation is very bad.''

The U.N. convoy came under sniper fire earlier in the day. It's unclear by which side in Syria's protracted civil war.

U.S. Secretary of State John Kerry said evidence of the massive chemical attack was "undeniable" -- signaling the U.S. was edging closer to a possible military response.

(SOUNDBITE) U.S. SECRETARY OF STATE JOHN KERRY

"Make no mistake. President Obama believes there must be acc