US officials expected the violence to drop after Iraqis regained control of their government on June 28, but attacks against US and Iraqi forces have averaged 49 a day since then, compared to 52 a day in the month before the transfer, USA Today reported Monday.
President George W. Bush has said US forces will stay in the country to assist Iraqi security forces as long as necessary.
"If we have the political will and stamina to stay, I could see this going on for 10 years," Randolph Gangle, head of the Marine Corps' Center for Emerging Threats and Opportunities, was quoted as saying.
A more optimistic view came from Colonel Dusty Rhoades, a Marine intelligence officer in Iraq.
"If we can stay the course over here for another year or so, the insurgency will wear itself out. The US military is currently in a position where it is militarily impossible for us to lose, but only an Iraqi government can totally win."
The Marine Corps is updating its manual on counterinsurgency, the "Small Wars Manual," based on its experience in Iraq, where the conflict stands out for the array of different groups involved.
"Here you have a whole hodgepodge of differing groups," Gangle said.
And the US Army is hiring a research group, The Dupuy Institute, to try to figure out how long the fighting may last. The group will study guerrilla conflicts in Greece, Indonesia, Malaysia and elsewhere to see how they pertain to the war in Iraq, the report said.