Heavy black smoke poured from the burning wreckage of an armored personnel carrier, known as a Rhino, in Kabul after the bomber struck. The bus had been sandwiched in the middle of a convoy of mine-resistant military vehicles when it was hit along a four-lane highway often used by foreign military trainers in the southwestern part of Kabul.
The landmark Darulaman Palace, the bombed-out seat of former Afghan kings, was the backdrop to the chaotic scene: Shrapnel, twisted pieces of metal and charred human remains littered the street.
U.S. soldiers wept as they pulled bodies from the debris, said Noor Ahmad, a witness at the scene. One coalition soldier was choking inside the burned bus, he said.
"The bottom half of his body was burned," Ahmad said.
The Afghan Ministry of Interior said four Afghans, including two children, also died in Saturday's attack. Eight other Afghans, including two children, were wounded, said Kabir Amiri, head of Kabul hospitals.
In all, there were three attacks Saturday against NATO and Afghan forces across the country.
"It's a huge loss," said U.S. Ambassador Ryan Crocker. "Our deepest sympathies go out to their comrades and families, but it will not deter us from our mission. It's a shock, but we will not let these guys win."
Just a day earlier, the Pentagon issued a progress report saying that the number of enemy-initiated attacks in Afghanistan was trending downward. Since May of this year, the monthly number of these attacks has been lower than the same month in 2010, something not seen since 2007, it said.
However, the Pentagon also noted that the insurgency's safe havens in Pakistan and the limited capacity of the Afghan government could jeopardize efforts to turn security gains on the battlefield, primarily in the south, into long-term stability in Afghanistan.
Saturday's attack broke a relative lull in the Afghan capital, which has experienced a number of attacks in recent years that are often blamed on the Haqqani network, an al-Qaida and Taliban-linked movement that operates out of Pakistan.
The Taliban identified the bomber as Abdul Rahman and said he was driving a Toyota Land Cruiser SUV containing 1,540 pounds (700 kilograms) of explosives and targeting foreigners providing training for Afghan police. The Taliban, who frequently exaggerate casualty claims, said that 25 people were killed by the blast.