The explosion was powerful enough to rattle windows across Kabul. It left bodies strewed along the street and one of the American vehicles — an armored Chevrolet Suburban that weighed nearly five tons — lying in ruins more than 30 feet from the blast site.

Hezb-i-Islami, a relatively small insurgent faction that often competes with the Taliban for influence, claimed responsibility for the attack, which also wounded more than three dozen Afghans. Haroon Zarghon, the group’s spokesman, reached by telephone in Pakistan, said the bombing was carried out by a 24-year-old man who had grown up south of Kabul.

More attacks against Americans will come soon, Mr. Zarghon added, saying that Hezb-i-Islami was dismayed by the current talks between Afghanistan and the United States about a long-term security deal under which thousands of American soldiers could be based in Afghanistan for years to come.

“When Hezb-i-Islami Afghanistan realized that American invaders have the devil intention of staying in Afghanistan, we decided to step up our attack on Americans in Afghanistan,” he said.

Whether Hezb-i-Islami — or the Taliban, for that matter — can regularly strike Americans in Kabul remains to be seen. Thursday’s bombing was the first significant attack in months on a Western target in Kabul, despite repeated efforts by insurgents to carry out a major strike in the city.

According to Afghan and American officials, the insurgents have found their efforts stymied by the myriad layers of security that protect Kabul, from street-level police officers staffing checkpoints to Afghan and foreign Special Operations soldiers raiding homes and businesses nearly every night.

The aftermath of Thursday’s bombing, though, provided a gory reminder of the war that still grips much of Afghanistan, and of the fact that only so much can be done to keep it from spilling over into Kabul, especially when the insurgents easily blend into the population.

The car bomber’s vehicle, a white Corolla, is probably the most commonly seen car in Afghanistan, and the driver shot out of a side street, a fairly standard maneuver on Kabul’s chaotic and crowded roads. It is likely that the Americans who were targeted had little or no time to react once the threat became apparent, if they were able to spot it at all.

The explosion left a deep crater in the road and cracks in the mud-brick shops that line the street. One of the two American Suburbans was reduced to a mangled heap of charred metal, while the other was launched into the air and blown down the street.

Human remains and bits of metal and plastic and other material from the cars were scattered for hundreds of feet around the site of the attack. Blown-apart ration packets carried by soldiers could be seen, along with a partly burned iPhone.

The United States-led coalition, in a brief statement, said two service members and four contractors had been killed. It did not specify their nationalities, though Afghan officials said they were all Americans.

One witness, a man in his 40s who lives near where the explosives went off but would not give his name, said he was having breakfast with his family “when we heard a really loud boom, and then there was a fireball.” He added: “Our entire house was engulfed by smoke and dust. Glasses shattered, windows broke. Suddenly the daylight turned to darkness.”

He said he ran out to see a large generator outside an Afghan bank branch in flames and bodies littering the street. “Some bleeding, some with missing limbs, some black like coal,” he said, calling it a “dreadful scene.”

Capt. Faizullah, an Afghan Army commander at the scene, said an Afghan interpreter for the coalition had been killed along with the American advisers. The Americans worked with the intelligence department of the Defense Ministry, which is about half a mile from the scene of the attack, said Captain Faizullah, who, like many Afghans, uses a single name.

Kanishka Baktash, a spokesman for the Ministry of Public Health, said the blast also killed nine Afghan civilians. Many of their bodies were burned beyond recognition.

The attack was one of the deadliest this year against coalition forces. Foreign casualties have dropped off sharply in recent months, with Afghan forces taking on a greater front-line role and the coalition pulling back ahead of 2014, when NATO’s combat mission here is to end.

Hezb-i-Islami, the group that said it was behind Thursday’s attack, occupies a murky position in Afghanistan. It has a political wing that is among the most powerful factions in President Hamid Karzai’s government. Its militant wing has for years vied with the Taliban for influence in eastern Afghanistan and around Kabul, sometimes directly battling the other insurgents — and, on occasion, aiding Afghan and coalition forces against the Taliban.

But it also remains an active insurgent group. It previously claimed responsibility for a car bombing in Kabul in September. That attack targeted a minibus carrying flight crew members for planes contracted to fly for the United States Agency for International Development, the American government’s aid and development arm.