The force of the blast tossed the bus several yards, according to Afghan police at the scene. One witness, a taxi driver, said that the bus was lying on its side, completely blackened, and that it appeared to have crushed some of the dead Afghan civilians.

Only one person on the bus appeared to have survived the blast, according to NATO officials, but that person’s condition was considered grave.

The attack was one of four in the last two days on allied forces and government offices, including one on Saturday in which an Afghan soldier turned his weapon on the Australian troops he was working with, killing three.

In a sign of the continued tensions between Americans and their Afghan allies, President Hamid Karzai issued a statement condemning the Kabul attack, but did not note the loss of American military lives. “The enemies of Afghanistan carried out a dastardly and cowardly attack that caused sorrow for some Afghan families,” he said.

Mr. Karzai’s comments angered American officials in Kabul already bitter over his statement last Sunday that Afghanistan would back Pakistan in any war with the United States — one in a series of pronouncements that might be intended for domestic consumption but raise fears about Mr. Karzai’s steadfastness as an ally.

Saturday’s attack in Kabul was not only the war’s deadliest in the city for Americans, it was also the first time in a year and a half that United States forces suffered significant casualties from an insurgent attack in the normally safe capital. On May 18, 2010, five Americans and one Canadian were killed when their convoy was struck by a suicide bomber. That attack also took place on Darulaman Road, very close to the scene of Saturday’s carnage.