BAGHDAD — Vice President Joe 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.