Ayman al-Zawahri (undated video broadcast on pan-Arab satellite channel Al-Jazeera)

U.S. intelligence officials have released the complete text of a letter purportedly written by Osama bin Laden's top deputy to the leader of Iraq's insurgency.

The text expands on excerpts released last week, in which the writer - said to be al-Qaida's second-in-command Ayman al-Zawahiri - outlined a strategy to push U.S. forces out of Iraq and set up an Islamic state.

In the complete text released Tuesday, the writer urges insurgent leader Abu Musab al-Zarqawi to be ready to establish Islamic rule in Iraq once the Americans leave.

He warns Mr. al-Zarqawi about the killing of Shi'ite civilians, saying it confuses other Muslims and may cost him crucial public support.

The writer also says al-Qaida is facing "real danger" from Pakistani army operations along the Pakistani-Afghan border.

The letter has been posted on the website of Director of National Intelligence John Negroponte. A note on the site says U.S. officials are confident that the letter is authentic.

The note says the letter, which is dated July 9, was obtained during counter-terrorism operations in Iraq.