BAGHDAD, Iraq -- Vice President Joseph R. Biden yesterday warned Iraqi officials that the U.S. commitment to their country could end if it again were to descend into ethnic and sectarian violence.

Mr. Biden delivered the warning during a three-day visit to Iraq that began Thursday, just a few days after the United States formally withdrew most combat troops from Iraqi cities under a security agreement reached last year. It was the vice president's first visit since President Barack Obama asked him to take the lead on Iraq policy.

In meetings with senior Iraqi officials, including Prime Minister Nouri al-Maliki, Mr. Biden stressed that the United States would remain engaged in Iraq, even as its military role diminished amid a withdrawal expected to dramatically gather pace after parliamentary elections in January 2010.

But a senior administration official briefing journalists said he made that support contingent on Iraqi progress in resolving long-standing conflicts, some of which bedeviled Iraq even before the United States invaded in March 2003.

If "Iraq were to revert to sectarian violence or engage in ethnic violence, then that's not something that would make it likely that we would remain engaged because, one, the American people would have no interest in doing that, and, as he put it, neither would he nor the president," the official said. He added that there "wasn't any appetite to put Humpty Dumpty back together again if, by the action of people in Iraq, it fell apart."

The warning was a dramatic indication of the changing U.S. posture in Iraq, the foremost foreign policy concern of the Bush administration. The statements suggested that the Obama administration would absolve itself of responsibility if Iraq again descended into chaos, dragged down by the still-unresolved crises including border disputes between Kurds and Arabs and legislation for Iraq's oil resources.

Across Iraq, signs are rife of a diminishing U.S. role. Simply by virtue of the presence of 130,000 U.S. troops, the United States is sure to exercise decisive influence in the country. But the power it once wielded inside Baghdad has passed. Along with last month's pullout -- and a far larger one due to end by August 2010 -- staffing at the U.S. embassy will be reduced over the year as well.

Even the interaction of officials seems to have changed. Earlier in the day yesterday, Biden aides huddled with advisers to Ayad al-Samarraie, the speaker of Iraq's parliament, trying to figure out a time when the two men could meet.

A Samarraie aide told the vice president's staff that the speaker had no more than 30 minutes to spare for the meeting. "That is the best I can do," the aide said, in a conversation overheard at a presidential palace, as workers vainly tried to clean the aftermath of a sandstorm that has buffeted Baghdad for almost a week.

Some lawmakers were even irritated by the secretive nature of Mr. Biden's visit. "They have to treat Iraq as a sovereign country," lawmaker Mahmoud Othman said. "They should have let us know, and they should be welcomed like any other leader."

During the day, with the capital cloaked in the sickly yellow glow of the sandstorm, which hampered Mr. Biden's travel, the vice president met with Ambassador Christopher Hill and the U.S. military commander in Iraq, Gen. Ray Odierno. He then met with senior Iraqi leaders and, at least publicly, stressed that the United States remained fully engaged.

"President Obama asked me to return with a message -- that the United States is committed to Iraq's progress and success," Mr. Biden said in a statement aired by Iraqi state television after his meeting with Mr. Maliki.

"We are looking forward to strengthening our relationship," Mr. Maliki added.

In Mr. Biden's last visit to Iraq in January, before Mr. Obama's inauguration, Iraqi and U.S. officials said he had delivered another message: U.S. patience had its limits, and Iraqi politicians would have to make a more concerted attempt to resolve their conflicts, particularly over the disputed border and the future of the contested northern city of Kirkuk.