US officials in Iraq say they have uncovered what they believe is a plot by a militant linked to al-Qaeda to foment sectarian violence there.
Abu Musab al-Zarqawi is under a death sentence in his native Jordan
The Americans seized a memo thought to be from Abu Musab al-Zarqawi, a suspected Jordanian militant.
The message laments the failure to expel US troops from Iraq - but suggests igniting the Shia-Sunni conflict could rescue the resistance.
Iraq's majority Shias were persecuted under Saddam Hussein, a Sunni.
US officials say the message was contained on a computer disk confiscated during a raid on a Baghdad house in mid-January.
That coincides with the arrest in Iraq of Hassan Gul, a Pakistani man suspected of working as a courier for al-Qaeda.
"There is clearly a plan on the part of outsiders to come into this country and spark civil war, breed sectarian violence and try to expose fissures in the society," US military spokesman Brigadier-General Mark Kimmit told a news conference in the Iraqi capital on Monday.
The 17-page document, parts of which were seen by the New York Times, was apparently intended for the al-Qaeda leadership and is believed to say attacks on Shia targets could create a backlash against the Sunnis.
This, in turn, would radicalise the Sunnis, driving fresh recruits into al-Qaeda's ranks.
"If we succeed in dragging them [the Shia] into a sectarian war, this will awaken the sleepy Sunnis who are fearful of destruction and death at the hands of the Shia," the document reads.
The author of the message admits the resistance against US occupation is struggling to recruit Iraqis and says this campaign must start before the "zero hour", when the US hands over power to an Iraqi administration in June.
US Secretary of State Colin Powell in Washington said the letter added "credence" to US pre-war claims about connections between al-Qaeda and the Iraqi leadership.
While addressing the UN before the Iraq war, Mr Powell said Mr Zarqawi was in Iraq and his presence showed Saddam Hussein's regime was courting al-Qaeda.
The killing of cleric Ayatollah al-Hakim angered Shias last year
The charge was denied by the Iraqi government and by Ansar al-Islam, the Islamist militant group in northern Iraq with whom Mr Zarqawi is said to have been allied.
The BBC's Barbara Plett in Baghdad says not all US intelligence statements about al-Qaeda's role in Iraq have been substantiated.
But if this report is true, she says, it would explain a number of sectarian incidents that do not fit the routine pattern of attacks against US soldiers and Iraqi police.
These include the bombing of mosques and the killing of the Shia religious leader, Ayatollah Mohammed Baqr al-Hakim, last August.
Mr Zarqawi has been accused by the US of masterminding a number of attacks in Iraq.
The author of the intercepted message admits responsibility for "25 of these operations, some of them against the Shia and their leaders, the Americans and their military and the police".