BAGHDAD, July 4 (Reuters) - U.S. Vice President Joe Biden met U.S. troops preparing to mark their Independence Day holiday on Saturday, on the third day of a visit he has used to urge Iraqi politicians to do more to reconcile rival factions.

Biden, who U.S. President Barack Obama has asked to take a leading role in coordinating U.S. Iraq policy, also presided over a naturalisation ceremony for 237 soldiers from 59 countries taking an oath of U.S. citizenship.

Baghdad remained cloaked in a sandstorm that grounded flights and forced Biden to scrap a planned trip to Iraq's largely autonomous Kurdistan region to meet Iraqi President Jalal Talabani, a Kurd, and Kurdistan President Massoud Barzani.

U.S. forces pulled out of Iraq's towns and cities this week under the terms of a bilateral security pact that paves the way for a full U.S. withdrawal by 2012, raising concerns Iraq has not made enough political progress to prevent more fighting.

Violence has dropped sharply but there remain deep and worrying divisions between Iraq's majority Shi'ite Arabs, Sunni Arabs, ethnic Kurds and other minorities, which some fear could threaten Iraq's future stability.

But as the United States turns its attention to Afghanistan and moves ahead with a plan to end the unpopular Iraq war, Obama has pledged to halt combat operations here in August 2010.

During his meetings with Prime Minister Nuri al-Maliki on Friday, Biden warned that resurgent sectarian or ethnic violence in Iraq was "not something that would make it likely that we'd stay engaged," a U.S. official told reporters on Friday.

He added that in Washington, there is no longer "any appetite to put Humpty Dumpty back together again", should Iraq fall apart.

After the citizenship ceremony, Biden went to the main mess hall at Camp Victory, where he met privately with the Delaware National Guard unit where his son, Beau, serves.

He walked through the main cafeteria, grasping soldiers' hands and giving them hugs as they surveyed a July 4th menu of barbecued ribs, hamburgers and elaborately decorated cakes. (Reporting by Andrew Quinn; Writing by Tim Cocks and Andrew Quinn; editing by Myra MacDonald)

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