WASHINGTON (AP) -- Defense officials say four U.S. troops were killed Tuesday at or near Bagram Air Base in Afghanistan.

Officials say the four were killed by indirect fire, likely a mortar or rocket, but they had no other details.

The officials spoke on condition of anonymity because they were not authorized to provide details on the deaths.

The attack comes as U.S. and allied forces formally handed over control of the country's security to the Afghan army and police in a ceremony in Kabul. The transition to Afghan-led security means U.S. and other foreign combat troops will not be directly carrying the fight to the insurgency, but will advise and back up the Afghan forces as needed with air support and medical evacuations.