John Kerry has accused George W. Bush of creating a "crisis of historic proportion" with his invasion of Iraq, leaving Americans with the prospect of a war with no end in sight.

"In Iraq, this [Bush] administration has consistently overpromised and underperformed," Kerry told an audience at New York University. "This policy has been plagued by a lack of planning, an absence of candour, arrogance and outright incompetence."

"The satisfaction we take in his downfall does not hide this fact —we have traded a dictator for a chaos that has left America less secure," he said.

"After insulting allies and shredding alliances, this president may not have the trust and confidence to bring others to our side in Iraq," he said.

"But we cannot hope to succeed unless we rebuild and lead strong alliances so that other nations share the burden with us."