The alleged plan is outlined in a 17-page memo that U.S. forces confiscated from an al Qaeda suspect in Iraq, The New York Times reported. The paper said its reporter viewed the Arabic document and a military translation on Sunday.
The Times says the document is the strongest evidence to date of post-invasion contacts between extremists in Iraq and al Qaeda.
The document expresses frustration over efforts to force the United States out of Iraq and suggests that attacks on Shiites would prompt Sunni retaliation and a cycle of widening violence, the Times said.
"It is the only way to prolong the duration of the fight between the infidels and us," the document says. "If we succeed in dragging them into a sectarian war, this will awaken the sleepy Sunnis who are fearful of destruction and death at the hands" of Shiites.
In other developments:
The intercepted al Qaeda memo and al-Sistani's calls for elections highlight the rising influence of religion on politics Iraq, a Muslim country where Saddam's regime enforced one-party secularism.
With Saddam's Baathists out of power, other secular parties are still staggering back from the grave where Saddam buried them and, as they jostle for a position in the government to come, may be too weak to compete with Islamic movements backed by popular clerics.
Secular movements are divided. The strongest — those with leaders sitting on Iraq's Governing Council — are led by former exiles mistrusted by many Iraqis. Others are headed by home-grown politicians who were last in the public eye during a bygone era: the 1950s and 1960s, when Iraq had a degree of parliamentary politics.
When the Baath Party seized power in 1968, Saddam put an end to those parties — their members were executed, imprisoned, exiled or forced to lie low.
It's still up in the air whether elections will take place any time soon — U.S. administrators hope to convince al-Sistani they aren't feasible ahead of the present June 30 deadline.
But if Iraqis do go to the ballot box, secular parties appear ill-positioned to challenge the Islamic movement. So some take a nuanced stance toward holding elections: Yes, but not just yet.
"I don't think the nationalist trend would get a good representation in the government," explained Abbas al-Jabiri, whose Arab Socialist Party is in a coalition of seven small, new factions.
The Times said that American forces arrested the man who had the al Qaeda on a computer disc and was taking it to Afghanistan to get it to al Qaeda's senior leaders.
U.S. authorities believe the memo was written by Abu Musab al-Zarqawi, a Jordanian who is suspected of having ties to al Qaeda, the Times reported. His presence in Iraq was one piece of evidence used by the Bush administration to allege a link between Saddam Hussein's regime and al Qaeda. There's been no confirmation of any such link.
The author of the document claimed he had directed about 25 suicide bombings inside Iraq, but said the resistance against the U.S. occupation was struggling to recruit Iraqis and to combat American troops.
The memo even offers a kind of praise for U.S. forces, saying "America, however, has no intention of leaving no matter how many wounded nor how bloody it becomes."