Warnings by top US generals of a growing threat of civil war in Iraq are confronting US policymakers with somber questions about the future of a costly three-year-old mission to stabilize the country.
Analyst said civil war would force the US to choose between withdrawing its troops or take sides in what could become a wider regional conflict.
US officials insist the violence between Shiites and Sunnis is still confined mainly to Baghdad and is not yet "a classic civil war."
But the sectarian violence is "as bad as I've seen it, in Baghdad in particular," and civil war is a possibility, the top US general in the Middle East, General John Abizaid, warned Congress on Thursday.
His assessment was only the latest sign of high level concern that the situation has drifted rapidly toward civil war since national elections last December.
Britain's outgoing ambassador to Iraq is reported to have advised his government that "a low intensity civil war" was more likely than a transition to a stable democracy.
Last week, US commanders ordered more US troops to Baghdad after a wave of kidnappings, assassinations, massacres and bombings engulfed an Iraqi-led effort to secure the capital.
Abizaid said the situation in Baghdad was at a "decisive" juncture but he believed that Iraqis would ultimately compromise "because the alternative is so stark."
Senators wanted to know what civil war would mean for the mission of the 133,000 US troops in Iraq.
"I'm reluctant to speculate about that," US Defense Secretary Donald Rumsfeld said. "It could lead to a discussion that suggests that we presume that's going to happen."
Senator John Warner, the Republican chairman of the Senate Armed Services Committee, warned that if Iraq does descend into civil war the administration may have to seek a new mandate from the US Congress.
"If that were to come about, I think the American people would ask, `Well, which side are we going to fight on? Or do we fight both? And did we send our troops there to do that? We thought we sent them there to liberate the Iraqis, which we have done at a great sacrifice, 2,500-plus,'" he said in an interview with PBS television.
Independent analysts said civil war was not a foregone conclusion and that military action and political moves could yet contain and suppress the violence.
Much depends, though, on how susceptible an already weak political center in Iraq is to pressure from both Sunni and Shiite extremists behind the violence.
If it leads to the collapse of Iraq's central government and security forces along sectarian lines, the US mission would become untenable, some analysts believe.
"Unsettling though it may sound, the United States could end up with no alternative to pulling out of a country that had degenerated into chaos," said Loren Thompson, director of the Lexington Institute, a Washington group that specializes in military analysis.
"It seems improbable but our role in Iraq is to build democracy so if the center doesn't hold, there is nothing left to defend," he said.
A withdrawal of US forces in the midst of a civil war would be "a huge defeat for American diplomacy, in fact possibly the greatest defeat ever," he said.
"However, there is no point in sticking around to preside over a meltdown. If a country is going to divide along sectarian lines it would be very dubious strategy to try to prevent a natural process from unfolding," he said.