WASHINGTON -- A Pentagon official says all four of the service members killed in a bomb attack in southern Afghanistan were Americans.

The NATO international military command in Kabul provided no details on Monday's attack, including the nationalities of the dead.

But in Washington a defense official said they were American, and they were hit by an improvised explosive device.

The official spoke on condition of anonymity because relatives of those killed were still being notified.

The latest deaths make a total of 16 NATO service members killed so far this month, and 167 so far this year.