The letter, which the US says is from Ayman al-Zawahiri - reportedly Osama bin Laden's top lieutenant - to Abu Musab al-Zarqawi, al-Qaida's top leader in Iraq, also complained about communications, unity of command and funding problems to the extent that he asked al-Zarqawi for money, Pentagon spokesman Bryan Whitman said on Friday.
Al-Zawahiri "says they should avoid tactics such as bombing mosques and slaughtering hostages in order to not alienate the masses", Whitman said.
But an internet posting of an audio recording attributed to al-Zarqawi has him saying that targeting "infidels", regardless of whether they are civilians or soldiers, was legitimate.
The US has not released a copy of the al-Zawahiri letter, and officials declined to quote it verbatim apparently because of the difficulty of translating the Arabic version into English.
Its existence was disclosed to certain US news organisations on Thursday after President George Bush gave a major speech aimed at shoring up eroding US support for the war against al-Qaida.
Whitman said the letter laid out in a blunt manner al-Qaida's broader strategy of driving US forces from Iraq, establishing an Islamic state across the Muslim world and ultimately destroying Israel.
"What this letter demonstrates is that they also know the importance of Iraq," he said. "They understand how important Iraq is to their effort, as being central to their efforts right now, to their strategy and to their long-term plans."
"It also demonstrates that among the terrorist leaders there are differences of opinion concerning the terrorist tactics, but it also shows unity in terms of their overall objectives"
The 6000-word letter was dated in July and obtained sometime after that, apparently by the US military.
It was unclear whether it was intercepted electronically or captured in a raid. But it had been received by al-Zarqawi, a defence official said.
"It's believed to be authentic," Whitman added.
As described by US officials, the al-Zawahiri letter buttresses Bush's view that the United States faces a long war against a determined enemy, and that Iraq is now at the centre of that struggle.
Differences of opinion
Whitman said the letter demonstrated that the Islamic extremists "have the overall goal of the withdrawal of US forces from Iraq, establishment of an Islamic state centred on Iraq and its neighbouring nations and eventually focused on Israel".
The letter purportedly hits out at
the practice of killing hostages
"It also demonstrates that among the terrorist leaders there are differences of opinion concerning the terrorist tactics, but it also shows unity in terms of their overall objectives," he said.
The letter also noted that the majority of the battle in Iraq was conducted through the media and that al-Zarqawi should conduct his actions outside the view of the Western media.
The impression left was that "the sensationalised killings that
alienate the masses that are covered by the Western press are harmful to what they are trying to do", Whitman said.
Al-Zarqawi: Infidels fair game
But in an audiotape broadcast on the internet early on Saturday, al-Zarqawi purportedly says Islam permits the killing of "infidel" civilians.
"In Islam, making the difference is not based on civilians and military, but on the basis of Muslims and infidels," said the voice attributed to the fugitive leader who has a $25-million price on his head.
"The Muslim's blood cannot be spilled whatever his work or
Al-Zarqawi tape says infidel
civilians are legitimate targets
place, while spilling the blood of the infidel, whatever his work or place, is authorised if he is not trustworthy," said the tape, whose veracity, as with most internet postings, could not be determined.
Al-Zarqawi's al-Qaida in the Land of Two Rivers has claimed, often in internet statements, responsibility for some of the most gruesome attacks in Iraq, including the beheadings of foreign hostages and Iraqis.
Al-Zawahiri's letter also said they had lost key leaders, that they had, to a large degree, resigned themselves to defeat in Afghanistan and that their lines of communication and sources of funding had been "severely disrupted", he said.
Al-Zawahiri also purportedly tells al-Zarqawi that, because of the disruptions, "they don't know what's going on so much, and that they have this lack of support, suggesting that perhaps Zarqawi can help Zawahiri with money", Whitman said.
"Zawahiri, again talking to Zarqawi, says there appears to be a lack of unity of effort, and perhaps even a unity of command," the spokesman added.