Five Americans and an Afghan doctor have been killed by a car bomber in Zabul province in southern Afghanistan.
United States Secretary of State John Kerry said the Americans, three soldiers and two US government officials, were in Zabul to donate books to a school in Qalat.
He paid particular tribute to the State Department foreign service officer who died, describing her as young, smart and capable.
Another American civilian was killed in a separate attack in eastern Afghanistan.
The strikes came as US General Martin Dempsey arrived in Kabul on an unannounced visit on Saturday.
It was the deadliest day for the coalition since 12 July, 2012, when two separate attacks killed seven soldiers, also in southern and eastern Afghanistan.
The visit of General Dempsey, chairman of the Joint Chiefs of Staff, is to assess the level of training the United States will need to provide Afghan forces following the withdrawal of 100,000 remaining troops in 2014.