A security firm has confirmed that four civilian contractors killed in a suicide car bombing in Afghanistan were Americans.

Thursday's bombing in Kabul also killed two American soldiers and nine Afghan civilians, including two children.

DynCorp International says four of its employees were working with U.S. forces training the Afghan military when the blast occurred.

An Islamic militant group, Hizb-e-Islami, claimed responsibility for the attack, saying it had stalked the convoy for weeks.

It was the bloodiest attack in the Afghan capital since March 9, when suicide bombers struck while U.S. Secretary of Defense Chuck Hagel was visiting.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

A suicide bomber rammed his car into a U.S. convoy in Kabul on Thursday, killing at least 15 people including two American soldiers and four civilian contractors, officials said. The brazen attack made May the deadliest month this year for coalition forces.

Cmdr. Bill Speaks, a spokesman for the U.S. Defense Secretary, confirmed that the two soldiers from the NATO military coalition who were killed in the bombing were Americans. He would not comment on the nationalities of the civilian contractors.

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."

An Islamic militant group, Hizb-e-Islami, claimed responsibility for the early morning bombing, saying its new "martyrdom" unit stalked the convoy for weeks. The announcement could signal escalation by the movement, based in northeastern Afghanistan, which has fought against the American-led coalition but is also a fierce rival of the Taliban.

Body parts littered the scene of the blast, and one U.S. vehicle was reduced to a mangled pile of metal. The explosion was powerful enough to rattle buildings on the other side of the city.

Nine Afghan civilians died in the blast, including two children, said Health Ministry spokesman Kanishka Beektash Torkystani, and 35 people were wounded.

May has become the deadliest month of the year for coalition forces, with a death toll of 18. Up to now international casualties have generally been falling along with the reduction in numbers of international soldiers. NATO plans to pull its combat forces out of Afghanistan next year.

The suicide bomber detonated with a car packed with explosives 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 shredded many of the victims.

"We saw two dead bodies of children on the ground," Amin said. "But the rest of the (shattered) bodies were scattered around."

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 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 configuration of their work has not yet been decided.

Hizb-e-Islami 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.

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Associated Press writer Kay Johnson contributed to this report.