The statement came as President George W Bush portrayed the unpopular war as a "must win" struggle against Al-Qaeda.
Public support for the war has bounced back slightly after plunging to an all-time low in May, but two of every three Americans still believe US forces should be brought home in part or entirely, according to a New York Times / CBS News poll.
'At least two more years'
The US military command in Baghdad, meanwhile, has drafted a plan that envisages a significant role for the US military in Iraq for at least two more years.
Officials were speaking ahead of anticipated September report on the current troops "surge."
It calls for restoring security in Baghdad and other local areas by the summer of 2008 and for achieving "sustainable security" across the country by the summer of 2009, The New York Times reported.
Jointly developed by General David Petraeus and Ambassador Ryan Crocker, the plan does not call for specific troop levels or timetables for the withdrawal of the 160,000-strong force, the newspaper said.
But it does anticipate a decline in the level of US forces after the current surge "runs its course" later this year or by early 2008, the newspaper said.
US forces would continue to train Iraqi security forces, partner with Iraqi units and fight extremist groups, it said.
"The article is accurate," said Colonel Steven Boylan, a spokesman for Petraeus, the US commander in Iraq, declining further comment.
A Pentagon spokesman also confirmed that a draft campaign plan has been prepared by the general's staff in Baghdad.
But spokesman Bryan Whitman said no decisions had been made on US force levels in Iraq and "any timelines and troop strengths would be tied to conditions on the ground."
Mr Petraeus and Mr Crocker are supposed to report to Congress in mid-September on the surge and the next steps in Iraq, but Congress is pushing for a rapid drawdown of forces with broad public backing.
Bush warns of Al-Qaeda
In a speech in South Carolina, Mr Bush sought to justify a longer US military involvement by tapping into public fear and hatred of Al-Qaeda.
"However difficult the fight is in Iraq, we must win it, and we can win it," Mr Bush said in a speech to uniformed military personnel.
"Surrendering the future of Iraq to Al-Qaeda would be a disaster to our country," he said.
"Some say that Iraq is not a part of the broader war on terror," he said.
"They claim that the organization called Al-Qaeda in Iraq is an Iraqi phenomenon, that it's independent of Osama bin Laden and it's not interested in attacking America. That would be news to Osama bin Laden."
Al-Qaeda in Iraq
A US intelligence estimate last week said Al-Qaeda's "core" is seeking to leverage its "affiliate," Al-Qaeda in Iraq, for attacks on the United States.
US intelligence and military officials, however, have said Al-Qaeda in Iraq is focused on the conflict in Iraq, and currently unlikely to export violence even if it does receive guidance from Al-Qaeda leaders in Pakistan.
Al-Qaeda in Iraq did not exist before the US invasion of Iraq but it is now a small but lethal subset of the larger Sunni insurgency in Iraq.
A US defense official said its core members are believed to number in the hundreds, about 90 percent of them Iraqis.
The New York Times / CBS News poll conducted over the weekend showed some movement in Mr Bush's favour since May, a low point when 61 percent of those polled said they were against the US invasion of Iraq, and 35 percent said they were for it.
In the latest poll, 51 percent were against the invasion and 42 percent in favour.