Vice President-elect Joe Biden promised U.S. support for Afghanistan''s struggle against terrorism, drugs and corruption, in surprise visit Sunday to a dangerous Taliban-stronghold area of Afghanistan.

The future of the region where al-Qaida planned the Sept. 11 attacks, Biden said, "affects us all."

Underscoring the difficulties of the fight to come, hundreds of militants crossed from eastern Afghanistan and attacked paramilitary forces in the lawless frontier of neighboring Pakistan on the same day Biden visited Afghanistan''s southern Kandahar and Helmand provinces.

Six troops and 40 insurgents were killed in the clash in Mohmand agency along the volatile border, said a military official speaking on condition of anonymity because he was not permitted to talk to the media.

President-elect Barack Obama has promised to end the war in Iraq and refocus U.S. military efforts on Afghanistan, where al-Qaida-linked militants and the Taliban are making a comeback after initial defeats in the U.S.-led invasion in 2001.

The U.S. is rushing up to 30,000 more troops to Afghanistan and some will go to these southern provinces.

Southern Afghanistan is one of the centers of the Afghan Taliban-led insurgency, which left some 6,400 people — mostly militants — dead in 2008 alone.