Al-Qaeda's second-in-command Ayman al-Zawahiri said a US congressional bill calling for a troop withdrawal from Iraq was proof of Washington's defeat, according to a web video posted on Saturday.

"This bill reflects American failure and frustration," Zawahiri said. "But this bill will deprive us of the opportunity to destroy the American forces which we have caught in a historic trap."

A White House spokesman declined to comment on the video, which comes four days after US President George Bush vetoed a $US124 billion ($150 billion) congressional war-spending measure that would have required a troop pullout from Iraq to begin by October 1.