The attack targeted two armored Chevrolet Suburban that were driving in Kabul Thursday morning, The New York Times reported.
A man in his 40s who lives near where the explosion happened said he was having breakfast with his family "when we heard a really loud boom, and then there was a fireball. Our entire house was engulfed by smoke and dust. Glasses shattered, windows broke. Suddenly the daylight turned to darkness."
NATO said two service members and four contractors were killed. Although their nationalities were not confirmed by the U.S.-led coalition, Afghan officials said they were all American.
Nine Afghan civilians were also killed in the blast, as well as an Afghan interpreter for the coalition forces, Afghan officials said.
More than 30 others were injured in the incident, officials said.
Hezbe Islamic, an insurgent group led by Gulbuddin Hekmatyar, claimed responsibility for the attack, CNN reported.
"We had planned this for days before we got a chance to target American advisers," Haroon Zarghon, a spokesman for the group, was quoted as saying.
In another suicide bomb attack Thursday, an Afghan shop owner died in southeast Paktika province, 120 miles south of Kabul, China's Xinhua News Agency reported.
Six other people were injured, including a pro-government tribal elder and two policemen, Xinhua said, quoting police official Daulat Khan Zadran.