The 9 July-dated letter, which US officials say was written by al-Qaida's second in command, Ayman al-Zawahiri, appears near its close to urge al-Zarqawi to send greetings to himself if visiting the Iraqi city of Falluja.

"My greetings to all the loved ones and please give me news of Karem and the rest of the folks I know," says an unedited English translation posted at http://www.dni.gov/, the website of US intelligence chief John Negroponte.

"And especially, by God, if by chance you're going to Falluja, send greetings to Abu Musab al-Zarqawi," it states.

Al-Zarqawi is the Jordanian-born leader of al-Qaida in Iraq. His organisation has said the letter is a fabrication.

A spokesman for Negroponte, who is the US director of national intelligence (DNI), acknowledged the greetings passage was confusing, but said the intelligence community was confident the letter was addressed to al-Zarqawi by al-Zawahiri.

"We don't know what to make of it. It's unclear," the Negroponte spokesman said.

"But we are absolutely confident that it was intended for Mr Zarqawi, based on a review by multiple agencies over a protracted period of time."

US officials have refused to disclose details of where, when or how the authorities came by the letter, or what methods have been used to determine its authenticity.

Credibility questioned

Some experts contend the strange passage undermines the letter's credibility.

"This would appear to be conclusive evidence that the DNI was mistaken and that the letter was written to someone other than Zarqawi," Steven Aftergood of the Federation of American Scientists said on Friday in his email intelligence newsletter, Secrecy News.

Aftergood cited an article in the online Slate magazine that called attention to the passage as well as the fact the letter was signed with the name Abu Muhammad.