US Vice President Joe Biden made a previously unannounced visit to Baghdad to meet Iraqi leaders and US military commanders just days after American troops withdrew from Iraqi towns and city centers. Biden flew in to Baghdad airport amid a sandstorm, which forced him to reschedule a planned evening visit to the US embassy.

His visit comes at a critical time in US-Iraqi relations. Washington is putting more pressure on Sunni, Shi'ite and Kurdish leaders to resolve disputes over oil revenues and regional boundaries that have stalled political reconciliation. Biden said he was optimistic about Iraq's future but that much remained to be done to achieve the political consensus needed to make progress after more than six years of chaos and bloodshed following the US-led invasion in 2003.

"I am optimistic because I do think that the Iraqis have become interested in their nationhood. They've become interested in the idea that they run their own lives," he told reporters on his plane. Biden's three-day visit comes after President Barack Obama appointed him to help coordinate Iraq policy as US officials lay the groundwork for a full withdrawal of US forces by 2012.

In a key step toward that, US troops handed over control of urban areas to Iraqi security forces this week under a bilateral Iraqi-US security pact. The sectarian warfare and insurgency since 2003 have receded dramatically over the past year, but attacks continue.

On the very day of the US pullback, a car bomb in the northern city of Kirkuk killed at least 30 people and wounded 65. Biden said Washington recognized many Iraqis were still anxious about security and violence, but said he believed the country was now moving beyond the danger of widespread sectarian or ethnic conflict.

White House officials said Biden would meet President Jalal Talabani and Prime Minister Nuri al-Maliki as well as visit US commanders and troops, marking the US 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 his son, Beau Biden, who is in Iraq with the Delaware National Guard in a unit that was deployed late last year.

Biden, who previously served as head of the Senate Foreign Relations committee, said his new role as Obama's lead official on Iraq was designed to help foster the cooperation needed among Iraqi factions to achieve "a stable, functioning government where there is neither sectarian violence nor ethnic violence. "This is on my watch again. This is my responsibility," he said.