BAGHDAD - U.S. Vice President Joe Biden arrived in Iraq on an unannounced visit on Tuesday to keep up U.S. pressure on Iraq's leaders to reach political compromises on thorny issues as U.S. combat troops prepare to go home.

It was Biden's second trip to Iraq in three months, and the visit signaled that the Obama administration is anxious to resolve long-standing disputes between Kurdish, Shi'ite and Sunni Arab communities over land and oil that U.S. officials fear could rip apart the country's fragile security.

Violence has dropped sharply in Iraq since the height of a wave of sectarian killings in 2006, due in part to a so-called surge of tens of thousands of U.S. troops, but the security gains have not been matched by much political progress.

Since 2006, Washington has pressed Iraq's Kurdish, and Shi'ite and Sunni Arab leaders, with little success, to put aside differences and compromise on issues such as a new oil law to manage the world's third-largest oil reserves.

But now with U.S. combat operations due to end in Iraq by August 2010, the United States is running out of time and influence among Iraq's leaders to achieve its goal -- to leave behind a relatively stable Iraq that can resist efforts by neighboring Shi'ite Iran to meddle in its affairs.

There is no longer any appetite in Washington for the Iraq war. The Obama administration is preoccupied with the deteriorating security situation in Afghanistan and rallying fading support among Americans and skeptical Democrats who control the U.S. Congress for the eight-year-old war there.

HUMPTY-DUMPTY

Biden warned Iraqi leaders on his last visit to Iraq that if there was renewed violence in Iraq, U.S. troops would not stay on to put Humpty-Dumpty back together again.

U.S. forces pulled out of Iraq's towns and cities on June 30 under a security pact that paves the way for a full U.S. withdrawal by the end of 2011.

The U.S. ambassador to Iraq, Chris Hill, assured U.S. lawmakers last week that a recent spate of bomb attacks blamed on al Qaeda would not upset the administration's timetable for withdrawing the 130,000 remaining U.S. troops.

Aides said Biden would meet the top U.S. commander in Iraq, General Ray Odierno, and Shi'ite Iraqi Prime Minister Nuri al-Maliki, who has sought to distance himself from Washington and portray himself as a nationalist leader strong on security ahead of elections next January.

Biden, the Obama administration's point man on Iraq, will also meet Iraqi President Jalal Talabani, a Kurd, and the leader of Iraq's semi-autonomous Kurdistan region, Masoud Barzani. A sandstorm prevented him from flying north to the Kurdish enclave on his last visit.

Northern Iraq is a key battleground between Baghdad's Arab-led government and Iraqi Kurdistan, which claim parts of the oil-producing north along its borders as the Kurds' ancestral homeland.

Massive oil reserves lie at the heart of the dispute, which is seen as a major threat to Iraqi security just as sectarian violence ebbs. U.S. and U.N. officials are trying to broker a compromise.

A recent spate of bombings in northern Iraq are the work of Islamist militants trying to exploit Kurd-Arab tensions to undermine security, the U.S. military says.

(Editing by Michael Christie)

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