Bin Laden's deputy tells militants to stop attacking civilians in Iraq

The instructions were conveyed to Zarqawi in a 6,000-word letter from Ayman al-Zawahari, Osama bin Laden's deputy, dated in early July. The document was apparently discovered by US forces in Iraq.

Pentagon officials who discussed the document with The New York Times are convinced it is authentic, setting out a grand design of al-Qa'ida for the Middle East region, of which the expulsion of American forces from Iraq is only a first step.

It became public on the day that six more US Marines were killed in Iraq by insurgent attacks, bringing the death toll of US servicemen to around 1,950.

In a striking echo of President George Bush's assertion that Iraq is the "central front" in the global war on terror, Zawahari described Iraq as the scene of "Islam's greatest battle of this era". But having forced the Americans to depart, the "mujahedin must not lay down their weapons". Instead, he declares, they must help set up a militant Islamic caliphate across Iraq, which would then be extended to cover Syria, Lebanon and Egypt.

The fourth and final stage of the campaign would be war against Israel. Written by a Sunni to a fellow Sunni, the letter accuses the Shia majority in Iraq of collaborating with the "enemies of Islam" - in other words the Americans. Nonetheless, Zawahari delivers a warning to al-Qa'ida's self-proclaimed chieftain in Iraq that the most brutal tactics, including suicide bombings, are counterproductive. Rather than uniting Muslims against the common Western enemy, they pitted them against each other, and Zawahiri implied, damaged the broader al-Qa'ida cause.

The content of the letter, including a request for more details on the state of the insurgency, implied that the two men were not in regular contact. This reinforces the belief that Zarqawi is very much his own man.