"We in al-Qaida declare that there is no truth to these claims, and they are baseless, except in the imagination of the politicians of the Black (White) House," according to the statement on a website known as a clearing house for al-Qaida material.
The statement was signed by Abu Maysara, who claims to be spokesman for al-Qaida in Iraq. It could not be authenticated.
"We call on Muslims not to pay attention to this cheap propaganda and to remember that the media will always be the infidels' sole weapon until the end of the battle," the statement said.
US officials said the letter to al-Qaida in Iraq leader Abu Musab al-Zarqawi, first disclosed by the Pentagon on Friday and released in full on Tuesday, was acquired during American operations in Iraq and dated 9 July.
In the letter, taking up 13 typed pages in its English translation, al-Qaida deputy leader Ayman al-Zawahiri recommended a four-stage expansion of the war that would take the fighting to neighbouring Muslim countries.
"It has always been my belief that the victory of Islam will never take place until a Muslim state is established ... in the heart of the Islamic world," al-Zawahiri wrote.
The letter laid out his long-term plan: The expulsion of US troops from Iraq, the establishment of an Islamic authority and the expansion of the war to Iraq's secular neighbours, including Lebanon, Jordan and Syria.
The letter, translated by the US government, also asked al-Zarqawi to provide financial support and urged him to avoid bombing mosques and slaughtering hostages to avoid alienating the masses.