The U.S. official emphasized details are continuing to unfold. A heavily damaged vehicle is believed to be an armored bus that was carrying U.S. troops from one base to another.

The attack caused a “number” of NATO and local Afghan casualties, ISAF said in a statement. Four Afghans, including two students, were also killed, said Hashmat Stanikzai, spokesman for Kabul’s police chief…

The deaths were the largest single-day U.S. loss in Afghanistan since the August crash in Afghanistan’s Wardak province that killed 38 people, including 17 Navy SEALS. That aircraft was brought down by an insurgent rocket-propelled grenade.

A Taliban spokesman confirmed Saturday’s attack in a text message, saying it killed “16 foreign soldiers, one civilian” and injured many others.