A roadside bomb killed four US troops in southern Afghanistan on Tuesday, inflicting the year's deadliest single attack on international forces a week after Washington set plans to send reinforcements. An Afghan civilian working with the Americans also died. The Americans were patrolling with Afghan soldiers when their vehicle struck a bomb Tuesday afternoon, the US military said in a statement. The military withheld identities of the dead and the attack's location pending notification of relatives. The previous deadliest attack on US troops this year was an explosion in Zabul province in January that killed three Americans. Twenty-nine US military personnel have died in Afghanistan this year, far surpassing the eight Americans killed in the first two months of 2008. A total of 654 have died since the US-led offensive that ousted Afghanistan's Taliban regime in late 2001.