The Pentagon report, released on Friday, claimed that Russia had given Saddam information on US troop movements after the US-led invasion that led to his ouster three years ago.
Russia's SVR foreign intelligence service has denied the report, which said the Russians collected information from sources in the US Central Command in Doha, Qatar, and then delivered it to Saddam.
Secretary of State Condoleezza Rice and White House National Security Advisor Stephen Hadley vowed to pursue the matter, as anger welled in Congress against the Russians for possibly putting US troops at risk.
Investigation promised
Republican Senator Pat Roberts, chairman of the Senate Intelligence Committee, said the matter would be looked into, and said lawmakers on the Senate Armed Services Committee were also planning to be more fully briefed.
Mr Roberts told CNN television that a mole inside the US military establishment "would be a very serious issue," but added: "I am not surprised, however, by Russian spying. I don't want to cause a major flap here, but that's what they do," he said.
Ms Rice said the US administration needed time to digest the information.
"Any implication that there were those from a foreign government who may have been passing information to the Iraqis prior to the invasion would be, of course, very worrying," she told CNN.
Earlier, Ms Rice told NBC television's Meet the Press program: "We would take very seriously any suggestion that this may have been done, maybe to the detriment of American forces."
"Definitely we will raise it with the Russian government," the chief US diplomat said. "I would hope the Russian government would take it seriously."
Ms Rice told the Fox News Sunday program that she had no reason to doubt or confirm the allegations but added: "I would not jump to the conclusion that, if indeed the reports are true, that it had to be Moscow-directed."