The unidentified diplomat told CNN the nine Americans killed included four U.S. troops. Others killed in the attack included two British civilians believed to be civilian contractors, a Kosovo national and a Canadian soldier. Afghan officials said four Afghans, including two students, also died in the incident, bringing the total death toll to 17.
It was earlier reported 13 Americans died in the Saturday attack when a car loaded with explosives rammed into the bus. But the diplomat told CNN that some of those in the bus were not on the passenger list, which led to confusion over the casualties.
"Afghan people are grieved by the NATO loss of lives and share the pain and sorrow with the families and friends of the troops killed," Afghan President Hamid Karzai said.
The Taliban claimed responsibility for the attack. The U.S.news network quoted an Afghan Interior Ministry spokesman as saying it was not clear how many people were wounded.
In other incidents Saturday, the International Security Assistance Force said NATO and Afghan troops killed 20 suspected Taliban insurgents following clashes in Nangarhar province, east of Kabul.