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".
"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".
'Tough job'
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.