U.S. Vice President Joe Biden made a previously unannounced visit to Baghdad on Thursday to meet Iraqi leaders and U.S. military commanders just days after U.S. troops withdrew from Iraqi towns and cities.

His visit comes at a critical time in U.S.-Iraqi relations. The Obama administration is putting more pressure on Sunni, Shi'ite and Kurdish leaders to resolve long-standing disputes over oil revenues and regional boundaries that have stalled political reconciliation.

Biden said he was optimistic about Iraq's future but much work remained.

Biden's three-day visit comes after President Barack Obama appointed him to help coordinate Iraq policy as U.S. officials lay the groundwork for a full withdrawal of U.S. forces by 2012.

In a key step towards that full withdrawal, U.S. troops handed over control of urban areas to Iraqi security forces this week.

"This is a moment when we have to make sure that the Iraqis don't take their eye off the ultimate prize," Biden said.

White House officials said Biden would meet President Jalal Talabani and Prime Minister Nuri al-Maliki as well as visit U.S. commanders and troops, marking the U.S. July 4 Independence Day holiday.

"He will discuss with Iraq's leaders the importance of achieving the political progress that is necessary to ensure the nation's long-term stability," the vice president's office said in a statement.

It was Biden's second trip to Iraq this year and his first as vice president. Obama visited Iraq in April.

Officials said Biden might meet up with his son, Beau Biden, who is in Iraq with the Delaware National Guard in a unit that was deployed late last year.