The top US commander in Iraq today told Congress that security gains in Iraq were "fragile and reversible" in a long-awaited report about the results of a year-old US troop build-up there.
Despite an improvement in security in parts of Iraq, Gen David Petraeus told the Senate Armed Services Committee that the situation in certain areas "is still unsatisfactory, and innumerable challenges remain".
"Moreover, as events in the past two weeks have reminded us, and as I have repeatedly cautioned, the progress made since last spring is fragile and reversible," he added.
All three contenders for the US presidency were among the senators expected to question Gen Petraeus, who said a process of withdrawing US combat troops from Iraq would be suspended after July for 45 days to allow commanders to evaluate the security situation.
"I recommended to my chain of command that we continue the draw-down of the surge combat forces and that, upon the withdrawal of the last surge brigade combat team in July, we undertake a 45-day period of consolidation and evaluation" Gen Petraeus said in his written testimony to a Senate panel.
The United States is withdrawing about 20,000 troops after a US "surge" of force last year.
Gen Petraeus and US Ambassador to Iraq Ryan Crocker faced a denunciation of US policy in Iraq from the committee chairman, Michigan Democratic Sen Carl Levin.
Gen Petraeus's plan to interrupt the US troop withdrawals, said Mr Levin, would simply be "the next page in a war plan with no exit strategy".
That decision could leave more than 130,000 US troops in Iraq though to the end of President George W Bush's term.
"We're no longer staring into the abyss of defeat, and we can now look ahead to the genuine process of success" - John McCain
Mr Levin said even the small political steps taken by the Iraqi government were in jeopardy because of the "incompetence and the excessively sectarian leadership" of Iraqi Prime Minister Nuri al-Maliki.
But Republican presidential nominee John McCain said he saw a genuine prospect of success in Iraq. "We're no longer staring into the abyss of defeat, and we can now look ahead to the genuine process of success," the Arizona senator said.
Meanwhile, Democratic Sen. Hillary Clinton has said the United States should begin withdrawing its troops from Iraq.
"I think it's time to begin an orderly process of withdrawing our troops, start rebuilding our military, and focusing on the challenges posed by Afghanistan, the global terrorist groups and other problems that confront Americans," said Clinton, a New York senator and former first lady who would be the first woman president.
In testimony over two days, Gen Petraeus and Mr Crocker will assess the uneven progress made in a year-long "surge" of force meant to create the calm for Iraqi politicians to advance legislation and factions to reconcile.
The United States now has 160,000 troops in Iraq. Under plans announced last year, the Pentagon is pulling about 20,000 troops out by mid-July, bringing the force level down to what it was before the surge.