The United States has intercepted a letter from Ayman al-Zawahiri (search), the No. 2 Al Qaeda leader, to Abu Musab al-Zarqawi (search), leader of the affiliated group Al Qaeda in Iraq, Pentagon officials told FOX News Thursday.

The letter says that the Al Qaeda leadership has have developed a detailed plan to create an Islamic state centered on Iraq, which would include neighboring countries. The plan also involves the destruction of Israel.

Pentagon spokesman Bryan Whitman would only say the letter had been intercepted recently, but would not say where in order not to reveal sources and methods.

Whitman told the Associated Press the letter demonstrates "that there is this detailed planning and intent on the part of the insurgents in Iraq to one day control that country and to really try to extend their extremism to neighboring countries. It demonstrates to me they clearly understand the importance and significance of the battle in Iraq right now."

According to the Pentagon, Zawahiri writes in the letter that Iraqi insurgents under Zarqawi's control should avoid attacking mosques or killing hostages to avoid alienating ordinary citizens.

Zarqawi recently declared war on Iraq's Shiite (search) majority, and hundreds of Shiites have been killed in bombings and shootings in the past few months.

The main body of Al Qaeda, to which Zarqawi pledged allegiance last year, generally regards Shiites as apostates but has rarely, if ever, acted against them.

Whitman did not make the letter available, but said it also revealed that the main Al Qaeda leadership had lost many key leaders and sources of financing that Zawahiri asks Zarqawi for money.

The Pentagon says it believed the letter is authentic, but would not say whether it actually reached Zarqawi, or if Zarqawi responded.

It was released Thursday evening after a presidential speech, designed to revive public support for the war in Iraq, in which Bush said the insurgents want to establish and spread a radical Islamic empire, beginning with Iraq.

FOX News' Brett Baier and the Associated Press contributed to this report.