Among those killed were two U.S. soldiers and four American civilian contractors, according to an International Security Assistance Force statement.
Nine Afghan civilians, including two children, also were killed.
The suicide attack occurred early Thursday in Kabul, the deadliest since March 9, when Defense Secretary Chuck Hagel visited the city.
Afghan President Hamid Karzai called the attack “an inhumane and un-Islamic act perpetrated by the enemies who don’t want to see the Afghan Muslim nation to live in peace,” according to a government statement Thursday.
Hezb-e-Islami, a Muslim militant group that is not affiliated with the Taliban, claimed responsibility for the attack. It is headed by warlord Gulbuddin Hekmatyar, a former Afghan prime minister.
Hezb-e-Islami spokesman Haroon Zarghoon told The Associated Press that one of the movement’s operatives carried out the attack on two vehicles of U.S. advisers.
Mr. Zarghoon said the militant group has formed a new cell to carry out suicide attacks on U.S. and other coalition troops.
“The cell had been monitoring the movement and timing of the American convoy for a week and implemented the plan Thursday morning,” he said.
Last week, Mr. Karzai said he had agreed to an American request to keep nine U.S. bases in Afghanistan after the NATO combat mission ends in 2014, but the White House said it had no intentions of keeping any “permanent” U.S. bases here.
The Hezb-e-Islami militant group has not recently conducted an attack in the capital, but could be attempting to demonstrate its relevance as a player in post-2014 Afghanistan when the NATO combat mission is set to end, said the Washington Institute’s Daniel R. Green, an Afghanistan researcher.
Mr. Hekmatyar’s “recent attack is an attempt to show how relevant he is and burnish his anti-American and anti-coalition credentials so when we transition out, he’s someone the Afghan government has to contend with,” Mr. Green.
Hezb-e-Islami and the Taliban share the goal of driving out foreign troops and establishing a Muslim state.