President Bush said yesterday that the United States "will not run" from Iraq, directly challenging a prediction that U.S. officials have attributed to al Qaeda's No. 2 leader, who said American power in Iraq may rapidly collapse, as it did in Vietnam when the U.S. "ran and left their agents," creating a governmental vacuum.

In his weekly radio address, the president took the unusual step of replying to statements from a letter seized in military operations in Iraq that American intelligence officials said was from Ayman Zawahiri to Abu Musab Zarqawi, the terrorist commander in that country. The letter was released last Tuesday in Arabic and English by Director of National Intelligence John D. Negroponte.

U.S. intelligence officials, responding to questions about the letter's authenticity, have said it has been verified by multiple agencies over a period of time. They refuse to say where or how it was found, saying such a disclosure would expose intelligence sources and methods. They also will not say whether the 16-page Arabic version on the DNI's Web site (www.dni.gov) is a photocopy of the original or one rewritten for public release.

The letter attributed to Zawahiri advises Zarqawi that "things may develop faster than we imagine" in Iraq, and recalling the U.S. experience in Vietnam, it says, "We must be ready, starting now before events overtake us" to prepare for the departure of American forces.

Bush's response yesterday was, "The al Qaeda letter points to Vietnam as a model. . . . Al Qaeda believes that America can be made to run again. They are gravely mistaken. America will not run, and we will not forget our responsibilities."

Bush also said the Zawahiri letter shows "establishing al Qaeda's dominion over Iraq is the first step towards their larger goal of imposing Islamic radicalism across the broader Middle East," and that Osama bin Laden's group "intends to make Iraq a terrorist haven and a staging ground for attacks against other nations."

Bush's speech put on the public record for the first time what senior U.S. intelligence officials have been telling reporters on the condition of anonymity since Oct. 6. That is when the letter's existence was confirmed -- and several lines from it quoted -- to a small group of reporters on a day that Bush made a major speech on Iraq and the fight against terrorism. Five days later, another background briefing was held, and the DNI released the entire letter.

Two former senior intelligence officials, asking not to be identified because of the controversial nature of their views, questioned the DNI's role in what appeared to them to be a public relations effort by the administration. "Is this a role for the DNI?" one official asked. The other noted that the letter "provided useful confirmation of the views expressed by the president," and briefing reporters on the day of Bush's speech may show "the DNI is part of the [president's] team" and not independent.

Michael Scheuer, the former head of CIA's task force on the bin Laden network and author of two books critical of the administration's Iraq policy, said the letter's contents did not surprise him.

"On balance, I would say that the letter is authentic, though I would not be shocked if I was wrong," he said. "It is a different missive than those we are familiar with, but not different enough to shout forgery."

Scheuer described the letter's themes as "pure al Qaeda -- especially saving war against the Shia for after the Americans are gone and the apostate regimes are destroyed."

He also pointed to Zawahiri's stressing to Zarqawi that he needs the support of the Iraqi masses, and that beheadings and repeated bombing attacks against Shiite targets that kill civilians could lessen public support. His advice to kill adversaries with bullets, rather than in a videotaped beheadings, "is pure Zawahiri," Scheuer said.

Juan Cole, a professor at the University of Michigan at Ann Arbor who is fluent in Arabic, has questioned the letter's reliability, saying that several parts use language that would be used by a Shiite, while both Zawahiri and Zarqawi are Sunni. "I do not believe that an Egyptian like al-Zawahiri would use this phraseology at all," Cole said in a statement on his Web site. "But he certainly would not use it to open a letter to a Salafi," a reference to Zarqawi's Sunni fundamentalist beliefs, he added.

Government officials, who spoke on the condition that their names not be used, said that several native Arab speakers had reviewed the document and described it as "standard Arabic" formulation. They also said it "represents Zawahiri's views" but refused to say whether it was written by him or could have been dictated by him.

Cole said he has doubts about the authenticity of the letter, suggesting it could be the work of some Iraqi Shiite group attempting to manipulate the United States.

A Web site associated with Zarqawi's group last week raised questions about the letter's reliability, saying it had been doctored to be used as U.S. propaganda.