BAGHDAD - Vice President Biden celebrated the Fourth of July with his son and other American troops in Iraq on Saturday, a day after warning Iraqi leaders that U.S. assistance will be jeopardized if the country reverts to ethnic and sectarian violence.

Biden began Independence Day by greeting more than 200 U.S. soldiers from 59 countries who were becoming American citizens at a naturalization ceremony in a marble domed hall at one of Saddam Hussein's palaces at Camp Victory, the U.S. military headquarters on the outskirts of Baghdad.

He then had lunch with the 261st Theater Tactical Signal Brigade from Delaware, to which his son, Beau, belongs. Beau Biden stood in the back as his father greeted the troops. In telling the brigade about the naturalization ceremony, the vice president used some of his characteristic colorful language.

"We did it in Saddam's palace," he said. "That S.O.B. is rolling over in his grave right now."

Biden was wrapping up an unusually long three-day trip to Baghdad aimed at fostering political reconciliation, just days after U.S. combat troops withdrew from Iraqi cities as part of a security pact that calls for a full withdrawal by the end of 2011.

It was Biden's first visit as vice president and as Obama's new unofficial point man on Iraq, although he has been to the country several times as a senator. Biden planned to fly to the semiautonomous Kurdish region in northern Iraq later Saturday, but the trip was canceled due to heavy sandstorms.

On Friday, Biden pressed Iraqi Prime Minister Nouri al-Maliki and other political leaders to do more to bring Iraq's divided factions together, as concerns grow that a lack of political progress is fueling violence in Iraq.

"Iraq has traveled a great distance over the past year, but there is a hard road ahead if Iraq is going to find lasting peace and stability. It's not over yet," Biden said at a news conference with al-Maliki.

"There are still political steps that must be taken and Iraqis must use the political process to resolve their remaining differences and advance their national interest," he said. "We stand ready, if asked and if helpful, to help in that process."

While Biden stressed America's commitment to Iraq's progress in his public remarks, a senior U.S. official said the vice president warned the Iraqis that America won't be able to stay involved if Iraq falls back into the cycle of sectarian violence that pushed the country to the brink of civil war.

"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, speaking on condition of anonymity in order to discuss the confidential talks.

"There also wasn't any appetite to put Humpty Dumpty back together again if, by the action of people in Iraq, it fell apart," the official added.

Biden expressed particular concern that Arab-Kurdish tensions in the north might erupt into violence, according to a close al-Maliki aide.

Al-Maliki's adviser Yassin Majid said the prime minister told Biden that national reconciliation has always been on his government's agenda and that he had saved the country from plunging into civil war.

But he also said al-Maliki had no plans to embrace remnants of Saddam's ousted Baath Party, calling that "a red line," and he warned some groups want to exploit reconciliation for partisan gains.

Biden's visit and his new position overseeing the U.S. administration's Iraq policy reflect growing concern about a recent rise in violence after a series of bombings that killed scores of people.

A roadside bomb exploded on a street in Youssifiyah, south of Baghdad, on Saturday, killing one civilian and wounding five others, police said.

Al-Maliki, a Shiite, has been criticized for failing to take advantage of security gains to make progress in overcoming disputes between Shiites, Sunnis, Kurds and other groups as divisions deepen before Jan. 30 general elections.

Violence remains at low levels in Iraq compared with previous years, but U.S. and Iraqi officials have warned of more attacks after the city withdrawal deadline and in the run-up to the elections.

At least 447 Iraqi civilians were killed in June, double the toll from the previous month, according to an Associated Press tally.

Al-Maliki has called the U.S. withdrawal from cities a "great victory" and declared June 30 National Sovereignty Day.

On Friday, he expressed gratitude for the U.S. support.

"I have seen very clearly the keen determination from the vice president and his administration to support Iraq ... and a great readiness to give us a lending hand and support anywhere we ask," he said.

Associated Press writer Qassim Abdul-Zahra contributed to this story.

See for yourself

To see Vice President Joe Biden's speech, please click here.

‘We will write the next great chapter’

President Obama in his July 4 address said an "unyielding spirit is what defines us as Americans" and led people during the nation’s history to explore the West, persevere during the Depression and build a robust industrial economy.

To see the AP video, please click here.