WASHINGTON (Reuters) - A roadside bomb killed five U.S. troops in southern Afghanistan on Thursday, a Pentagon official told Reuters, less than a week after 30 American forces were killed in the deadliest incident involving U.S. troops in the Afghan war. The NATO-led International Security Assistance Force (ISAF) had previously said that five foreign troops had been killed, but declined to give their nationalities. Asked by Reuters, a Pentagon spokesman said they were American troops.