US Defence Secretary Donald Rumsfeld has warned it may be years before the insurgency in Iraq is defeated.
Insurgent attacks appear to be on the rise
Speaking on US television, Mr Rumsfeld said ultimately Iraq's own forces, rather than coalition troops, would beat the insurgents.
Earlier, Mr Rumsfeld said US officials in Iraq have had talks with leaders of the insurgency.
It comes amid growing concern in the US about rising casualties and warnings that the insurgency is strengthening.
Recent opinion polls in the United States have shown a considerable drop in support for the US-led invasion of Iraq.
President George W Bush is to make a prime-time address to the nation about the situation in Iraq on Tuesday.
More than 1,000 people - mostly Iraqis - have been killed since the new government was installed in April.
The US defence secretary told Fox News: "Insurgencies tend to go on five, six, eight, 10, 12 years.
"Coalition forces, foreign forces, are not going to repress that insurgency. We're going to create an environment that the Iraqi people and the Iraqi security forces can win against that insurgency."
Mr Rumsfeld warned that violence could escalate ahead of new elections for a permanent government, due in December.
Maintaining domestic support for a continuing role in Iraq was crucial, Mr Rumsfeld said, but he warned about paying too much attention to a myriad of opinion polls.
"If you start chasing polls, you're going to get seasick," he said.
"The task for the president and the government and the military leadership is to show that progress is being made, which it is."
The senior US general in the Middle East, John Abizaid, also called for Americans to remain calm.
"We don't need to fight this war looking over our shoulder worrying about the support back home."
The BBC's Ian Pannell in Washington says the White House, engaged in a public relations offensive, is worried by the rising casualties, the ongoing insurgency and waning domestic support.
The latest remarks by Mr Rumsfeld would suggest that managing expectations is now an important part of White House strategy, our correspondent says.
During a round of network TV interviews, Mr Rumsfeld made light of a report by a British newspaper that said US officials have secretly met with Iraqi insurgents.
Meetings go on "all the time", Mr Rumsfeld said, adding that Iraq's government often initiates contact.
"I would not make a big deal out of it.
Mr Rumsfeld denied a Sunday Times report that the US met with Ansar al-Sunna, which has carried out suicide bombings, and several other Islamist groups.
"There's no one negotiating with Zarqawi or the people that are out chopping people's heads off... but they're certainly reaching out continuously, and we help to facilitate those from time to time," Mr Rumsfeld said.
A statement allegedly from the leader of Ansar al-Sunna was also posted on the internet denying all contact.
"Jihad is the only way to restore dignity to this nation. Without this dignity, the nation will be shamed and defeated," the statement said.