An Islamic militant group claimed responsibility for the attack, saying it was carried out by a new suicide unit formed in response to reports that the U.S. plans to keep permanent bases and troops in Afghanistan even after the 2014 deadline for the end of the foreign combat mission. Hezb-e-Islami said its fighters had stalked the Americans for a week to learn their routine before striking.
It was the deadliest attack to rock the Afghan capital in more than two months and followed a series of other attacks against Americans that has made May the deadliest month for international forces this year. U.S.-led forces are increasingly leaving the fighting to their Afghan counterparts and focusing more on training mission in a bid to prepare the government forces to take over their own security after the international combat mission ends by the end of 2014.
The explosion was powerful enough to rattle buildings on the other side of the city, and left body parts scattered on the street.
Cmdr. Bill Speaks, a spokesman for the U.S. Defense Secretary, confirmed that two American soldiers were killed, while international security company DynCorp International said four of its American civilian contractors were among the dead. DynCorp International said its employees were working with U.S. forces training the Afghan military when the blast occurred.
Nine Afghan civilians also were killed, including two children, and 35 people were wounded, Health Ministry spokesman Kanishka Beektash Torkystani said.
The deaths pushed the monthly toll for the U.S.-led coalition to 18, making May the deadliest month so far this year. By comparison, 44 international troops were killed in the same period last year, reflecting the fact that the overall number of deaths has dropped as Afghan forces increasingly take the lead.
The suicide bomber detonated his explosives-laden car at about 8 a.m., Kabul provincial police spokesman Hashmad Stanakzi said. "The explosion was very big. It set the nearby buildings on fire," he said.
Kabul Deputy Police Chief Daud Amin said it was difficult to count the dead because the blast tore apart many of the bodies.
"We saw two dead bodies of children on the ground," Amin said. "But the rest of the (shattered) bodies were scattered around."
It was the bloodiest attack in the Afghan capital since March 9, when suicide bombers struck near the Afghan Defense Ministry while U.S. Secretary of Defense Chuck Hagel was visiting.
President Hamid Karzai condemned the attack, saying it was the work of "terrorists and enemies of Afghanistan's peace."
A spokesman for Hizb-e-Islami, Haroon Zarghoon, told The Associated Press that one of the movement's operatives carried out the attack on two vehicles of U.S. advisers.
Zarghoon says 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," Zarghoon said.
He said the cell was established in response to reports that the U.S. plans to keep permanent bases and troops in Afghanistan even after the NATO withdrawal.
The U.S. has said it wants no permanent bases in Afghanistan after 2014, but Afghan President Hamid Karzai raised eyebrows last week when he announced he had agreed to an American request to keep nine bases.
A small American force is expected to remain in the country to assist Afghans in keeping security, but the exact number or mission has not yet been decided.
Hizb-e-Islami, a fierce rival of the Taliban movement as well as the Americans, is headed by 65-year-old Gubuddin Hekmatyar, a former Afghan prime minister and onetime U.S. ally who is now listed as a terrorist by Washington. The militia has thousands of fighters and followers in the country's north and east.
Hekmatyar's government was heavily financed by the U.S. during Soviet occupation of Afghanistan in the 1980s. He is now is being hunted by Afghan and NATO troops. U.S. bombs have targeted his military chief, Kashmir Khan, in Kunar province in northeastern Afghanistan on the border with Pakistan. Khan was wounded but survived.
However, Hekmatyar's son-in-law has held peace talks with Karzai and American officials. In a further sign of the complexities of the Afghan insurgency, Hizb-e-Islami is also a rival to the Taliban insurgency, even though both movements share the goal of driving out foreign troops and establishing a state that would follow a strict interpretation of Islamic law.
Hekmatyar and the Taliban supreme leader, Mullah Omar, are said to be bitter personal enemies.
Thursday's attack was the second in eight months claimed by Hizb-e-Islami. In September, the militant group claimed responsibility when a female suicide car bomber killed least 12 people. At the time, Hizb-e-Islami said the attack was revenge for the film "Innocence of Muslims," which was made by an Egyptian-born American citizen and infuriated Muslims for its negative depiction of the Prophet Muhammad.
Associated Press writer Kay Johnson contributed to this report.
Copyright 2013 The Associated Press.