The US plans to begin withdrawing combat forces from Afghanistan in July but remains concerned that gains made in the nearly decade-long war could be reversible. There are also questions about the ability of Afghan security forces to take up the fight against a virulent insurgency.

NATO hopes Afghan forces will assume full responsibility for security by 2014.

Tensions have surfaced between the Obama administration and Karzai, whose government is plagued by charges of corruption. US officials have expressed grave concerns about how this is affecting efforts to stabilize and rebuild the country.

Just a month ago, Obama came to Afghanistan but did not meet with Karzai. The White House said that foul weather foiled plans to take Obama to the presidential palace in Kabul from the Bagram Air Field military base where he landed, and that technical difficulties prevented the two presidents from talking by secure videoconference.

Although the two leaders spoke briefly by telephone, the change of plans was seen by some in Karzai's circle as a snub. And it was unclear whether part of the reason for Biden's visit was to smooth things over with Karzai.

A senior US official traveling with Biden tried to present a united front, saying the US and Karzai are “very much on the same page” after a NATO summit in Lisbon in November and the December US review of its war strategy. The official said the visit comes at a “pivot point in our policy” with the US moving from a troop surge last year to the planned start of a pullout and it would give Biden an opportunity to discuss progress toward the transition.

The official spoke on condition of anonymity and was not authorized to talk to the media.

NATO and US officials insist they are making progress in quelling the insurgency. In an end-of-year review of strategy in Afghanistan, the Obama administration said the US had made advances in its push against the Taleban in the south — the insurgents' traditional stronghold - but acknowledged that “gains remain fragile and reversible.”

The White House said Biden, who last visited in January 2009, met Monday night with US Gen. David Petraeus, the commander of US and NATO forces in Afghanistan. He also plans to visit US troops and tour an Afghan Army training center.

Biden is also reported to be heading to Islamabad this week to deliver a message that the US will send more help to Pakistan, which US and Afghan officials see as a key partner in routing the Taleban.