US Defense Secretary Donald Rumsfeld says he cannot remember hearing the claim that Iraq could launch weapons of mass destruction within 45 minutes.
The 45-minute claim was publicised in the run-up to war
The claim was a part of the UK government's September 2002 dossier on Iraqi weapons of mass destruction arms dossier in the run-up to the war.
It came to British intelligence from an Iraqi military source, but its use in the dossier has caused controversy.
Tony Blair last week admitted he had not known full details of the claim.
He told MPs he had not been aware before the war that it referred only to battlefield weapons rather than long-range strategic missiles.
That admission prompted Conservative leader Michael Howard to call for the prime minister's resignation for failing to ask "basic questions" before the war.
Asked his view of the claim, Mr Rumsfeld told reporters at a Pentagon briefing: "I don't remember the statement being made, to be perfectly honest."
Last year, a committee of MPs said the way the claim was worded could have led to unhelpful speculation about its meaning.
Some newspapers printed banner headlines suggesting it meant British troops in Cyprus could be attacked with weapons of mass destruction within 45 minutes.
The claim became a central part of Lord Hutton's report into the death of government weapons expert Dr David Kelly.
BBC reporter Andrew Gilligan reported that Dr Kelly had cited the claim as an example of how the dossier was "sexed up" before its publication to make the case for war.
But Lord Hutton said the allegation that the government had embellished the dossier with intelligence it believed to be unreliable was "unfounded".