Two American soldiers among 15 killed in Kabul suicide bomb attack on Afghan Nato convoy

Muslim militant group Hizb-e-Islami have claimed responsibility for the early morning attack

Afghan security and media representatives gather at the scene of the explosion in Kabul (Getty Images)

US officials have confirmed that two American soldiers were among 15 people killed in a suicide bomb attack on a Nato convoy in the Afghan capital of Kabul today.

An Islamic militant group, Hizb-e-Islami, has already claimed responsibility for the attack, which also killed four contractors and nine Afghan civilians, saying it had formed a special "martyrdom unit" to attack foreign troops.

At least two children are thought to be among the dead.

Body parts could be seen littering the scene of the blast, which was reportedly large enough to rattle buildings on the other side of the city. One coalition vehicle was reduced to a mangled pile of metal.

Kabul provincial police spokesman Hashmad Stanakzi said the suicide bomber attacked at about 8 am with a car packed with explosives. "The explosion was very big. It set the nearby buildings on fire," he said.

The car reportedly hit the convoy as it passed the home of an Afghan politician. Two children are said to be among the dead.

The deputy chief of police, Daud Amin, said it was difficult to immediately estimate the number of dead because of the state of some of the victims.

"We saw two dead bodies of children on the ground," he said. "But the rest of the bodies were scattered in pieces around."

Two Kabul hospitals had earlier reported that at least six dead and 37 wounded were brought in from the scene, but that the toll could increase.

A spokesman for Hizb-e-Islami, Haroon Zarghoon, told The Associated Press news agency that one of the movement's operatives carried out the attack on what he described as two vehicles of American advisers.

He claimed that most of the American advisers were killed and their vehicles destroyed.

Zarghoon says the militant group has formed a new cell to carry out suicide attacks on US 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 claimed the cell was established in response to alleged American efforts to keep permanent bases and troops in Afghanistan even after the full Nato withdrawal.

The announcement could signal an escalation of actions from the northeastern-based group, which is a fierce rival of the Taliban.

The US has repeatedly said it wants no permanent bases in Afghanistan after most foreign troops withdraw by the end of 2014, but President Hamid Karzai raised eyebrows last week when he announced he had agreed to an American request to keep nine bases. A smaller 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 former warlord Gubuddin Hekmatyar, a former Afghan prime minister and one-time US ally who is now listed as a terrorist by Washington. The militia has thousands of fighters and followers across the country's north and east.

Hekmatyar, who was heavily financed by the US during the 1980s occupation of Afghanistan by the former Soviet Union, has been declared a terrorist and is being hunted by Afghan and Nato troops. US bombs have targeted his military chief Kashmir Khan in Kunar province in northeastern Afghanistan on the border with Pakistan.

However, Hekmatyar's son-in-law has also held peace talks with both Karzai and also 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, in particular, are said to share personal animosity.

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 abroad for its negative depiction of the Prophet Mohamad.