Vice President Joe Biden arrived Monday in Afghanistan on a trip the White House said was aimed at assessing progress toward handing over security to Afghan forces.
Biden's trip, which was unannounced, is his second to the country since January 2009, and comes amid mounting questions about the pace of progress in the nearly decade-long war.
The U.S. is to begin withdrawing combat forces from Afghanistan in July, and questions remain about the ability of the country's security forces to take up the fight. NATO hopes Afghan forces will assume full responsibility for security by 2014.
His visit comes roughly a month after President Barack Obama came to Afghanistan, telling U.S. forces in the country they are making progress in their mission to defeat terrorism. American troops comprise the bulk of the 140,000-strong NATO force that has been battling the Taliban.
NATO and U.S. officials have said they are making gains in quelling the virulent insurgency. But the Taliban has repeatedly shown their ability to regroup, despite constant offensives by the international coalition, and to carry out attacks across Afghanistan, seemingly at will.
Last year was the deadliest for NATO forces in the country, with over 700 troops killed.
In an end-of-year review of the U.S. strategy in Afghanistan, the Obama administration said the U.S. had made advances in its push against the Taliban in southern Afghanistan - the insurgents' traditional stronghold - but acknowledged that "gains remain fragile and reversible."
The White House said Biden was to meet with President Hamid Karzai, and visit with U.S. troops and civilian personnel. He was to also tour an Afghan Army training center.