KABUL - A roadside bomb killed two Americans, including a soldier, in an attack on a U.S. military convoy on the outskirts of the Afghan capital on Wednesday, officials said.

The attack came on a road leading to Bagram airfield to the north of Kabul. Bagram is the key base for U.S.-led troops in Afghanistan.

Violence in Afghanistan has surged in recent months to its highest levels since the Taliban were ousted after a U.S.-led invasion more than seven years ago.

An Afghan police source had said the latest blast had been caused by a suicide bomber but the same source later said it had in fact been a roadside bomb.

A spokesman for the U.S. military confirmed the attack, saying one American soldier and a U.S. civilian working for the military were killed.

Despite a fresh surge of foreign troops, the Taliban insurgency is spreading to other parts of the rugged countryside from the hardline Islamist group's traditional base of support in the south and east.

The Taliban rely on suicide attacks and roadside bombs as a major part of their campaign to drive out foreign forces.