A signed document found when Saddam Hussein was captured show him warning his followers to be wary of Arab militants, say US officials.
Saddam Hussein had several documents with him in hiding
Iraq's ex-leader seems to have feared foreigners might hijack the insurgency for their own ends, the officials say.
The New York Times, which got hold of the leaked document, said it challenges President Bush's claim of close links between Iraq and al-Qaeda.
It also suggests any co-operation in attacking the US-led coalition is new.
Saddam Hussein has been at an undisclosed location since he was found by US troops in a "spider hole" near his hometown of Tikrit on 14 December.
With him was a briefcase containing several documents.
Dan Senor, a senior advisor to Iraqi administrator Paul Bremer, told CNN on Wednesday that though Saddam had not been "terribly cooperative", his paperwork gave "a better sense of the structure of the insurgency and the financing behind it".
The leaked document is said to be a warning by Saddam to his followers to exercise caution about joining forces with foreigners.
"It's a document in which he urges his supporters to be cautious in dealing with jihadists," a US official told Reuters news agency.
"The implication is because he can't entirely trust these guys."
The ex-Iraqi leader was found in a hole in the ground
He said the document did not tell supporters to avoid dealing with foreign fighters, but just to be aware that they may not share the same goals.
A month before going to war, President Bush told Americans that Iraq had provided al-Qaeda with chemical and biological weapons training, and senior Iraqi intelligence officials had met al-Qaeda at least eight times in the early 1990s.
"Saddam Hussein has long-standing, direct and continuing ties to terrorist networks," he said.
Soon after the war ended, there were reports of foreign militants pouring across the borders seeking to make Iraq the new battlefield in the "jihad" or holy war.
However, recent reports suggest foreign fighters account for around only 10% of the insurgents in Iraq.
US spokesman Brigadier General Mark Kimmit recently told a press briefing that up to 300 foreigners were being held in Iraq.
"They're a threat, but the vast majority of the personnel that we have in detention for activities against Iraqi citizens remain personnel from this country," he said.