A roadside bomb killed four US troops patrolling in southern Afghanistan on Tuesday in the deadliest single attack on international forces this year. An Afghan civilian working with the Americans also died. The troops were patrolling with Afghan forces when their vehicle struck a bomb Tuesday afternoon, the US military said in a statement. The military did not release the location of the attack pending the notification of relatives. The previous deadliest attack against US forces this year was an explosion in Zabul province in January that killed three troops. Twenty-nine Americans troops have died in Afghanistan this year, far surpassing the eight US forces killed in the first two months of 2008. The US is increasing troop levels in Afghanistan. A record 38,000 US forces now operate in the country, many in Taliban strongholds in the dangerous south.