US President George W Bush says he is unhappy with the progress of the war in Iraq, admitting that a recent upsurge in violence is a "serious concern".
Civilians have faced "unspeakable violence", Mr Bush said
"I know many Americans are not satisfied with the situation in Iraq," he said. "I'm not satisfied either."
But Mr Bush ruled out a fixed timetable for withdrawing US troops, adding that victory there was vital to US security.
His comments come two weeks ahead of crucial US mid-term elections, and amid public unease over US policy in Iraq.
In what the BBC's Justin Webb in Washington says is an unusual departure from normal practice, the US president began the press conference at the White House by outlining recent setbacks in Iraq.
He said the deaths of 93 US troops and 300 Iraqi security personnel in the last month were of "serious concern" to him.
Civilians had suffered "unspeakable violence at the hands of the terrorists, insurgents, illegal militias, armed groups and criminals," he said.
He warned that if Iraq became a failed state, extremists could gain access to oil wealth and launch fresh attacks.
If the US was not successful in Iraq, he said, extremists could use it as a base from which to try to establish a "radical empire from Spain to Indonesia".
Mr Bush defended the role of Defence Secretary Donald Rumsfeld, who has faced a barrage of criticism for the way the Iraq war has been fought.
"I'm satisfied with how he's done all his jobs," Mr Bush said, calling Mr Rumsfeld "a smart, tough, capable administrator".
With opinion polls showing growing doubt over the US role in Iraq, Mr Bush said: "We cannot allow our dissatisfaction to turn into disillusionment about our purpose in this war."
"We're winning and we will win, unless we leave before the job is done," he said.
He said the administration's Iraq goals there had not changed, and that setting a fixed timetable for withdrawal "means defeat", but added that the US was constantly adapting its tactics.
In recent days senior Bush administration figures have increased pressure on the Iraqi government to rein in militias and death squads.
Mr Bush said that he was "making it clear that America's patience is not unlimited".
Mr Bush has faced criticism over his Iraq policy
But he said that at the same time, the US authorities would not "put more pressure on the Iraqi government than it can bear".
He also said the US would continue to support Iraqi Prime Minister Nouri Maliki as long as he continued to make "tough decisions".
Earlier, Mr Maliki pledged to "strike hard" at illegal militias, widely blamed for the growing sectarian violence, but insisted he was working to his own, not a US, timetable.
He was also critical of what he described as a lack of co-ordination with US-led forces over a raid in Baghdad's Sadr City that resulted in four deaths.
US aircraft were called in for back-up after Iraqi special forces came under fire in Sadr City, a stronghold of the Mehdi Army militia.
They were trying to capture the commander of an armed group who directed death squads, a US military statement said.
The deaths in the raid provoked anger in Sadr City
The military said "precision gunfire" was used "only to eliminate the enemy threat".
But Iraqi police said US troops shot at them while they were trying to take injured people to hospital.
At least four people were killed and about 15 injured, provoking anger in Sadr City.
Sectarian killings and tit-for-tat reprisals have becoming increasingly common in Baghdad and some other areas.
The Mehdi Army, a militia linked to radical Shia cleric Moqtada Sadr, has repeatedly been accused of involvement in death squads carrying out attacks on Sunnis.
Mr Maliki's fragile coalition government includes Shia parties - including Mr Sadr's - that have links to powerful militias.
BBC Middle East analyst Roger Hardy says the Iraqi prime minister is caught between his American ally and Shia public opinion, and has extremely limited room for manoeuvre.
He says it is now obvious to most people in the region that the relationship between the US and Mr Maliki's government is under strain.
Correspondents say the Bush administration is aware that public support for the war in Iraq is threatening painful election losses in mid-term elections due on 7 November, when Democrats will seek to wrest control of both houses of Congress from the Republican party.