"We announce today the end of a phase of the jihad (holy war) and the start of a new one ... to usher in the project of an Islamic caliphate and restore Islam's glory," Abu Hamza al-Muhajer said in the message.
"We swear we will not rest from our jihad ... before blowing up the filthiest house, dubbed the White House," he said.
"I tell the commander of the faithful, the honourable Sheikh Abu Omar al-Baghdadi: I have put 12,000 Al Qaeda fighters at the disposal of the Islamic state of Iraq," Abu Hamza al-Muhajer said, pledging allegiance to the emir of the "state" proclaimed last month.
The authenticity of the message attributed to Muhajer could not be independently confirmed.
An alliance of Sunni insurgents headed by the Iraq branch of Al Qaeda announced the creation of an independent Islamic emirate in Iraq in a video posted on the Internet on October 15 after parliament in Baghdad approved a federal constitution for the war-ravaged country.
In Friday's message, Muhajer lashed out at US President George W. Bush, describing him as "stupid," and said US forces occupying Iraq were preparing to cut and run.
"The enemy is now teetering under the blows of the mujahedeen ... and preparing to pack up and flee," he said.
"I thank the most stupid and worst president America -- the country of slaves and drugs -- has ever had for giving us this great historic opportunity," said the voice purported to be that of the Al Qaeda supremo in Iraq.
"Uncle Sam's dreams have evaporated in the land of oil and water," he said in a reference to Iraq.

Copyright AFP (Agence France-Presse), 2006