At least two soldiers and four contractors with NATO-led forces were killed along with nine Afghan civilians on Thursday when a suicide bomber in a car struck a military convoy in Kabul, officials said.

The bomber drove his explosives-laden vehicle into a convoy of the NATO-led International Security Assistance Force (ISAF), said Ayub Salangi, the Kabul police chief.

Nine Afghan civilians, including two schooolchildren, were killed and 39 were injured, said Baktash Turkistani, a Public Health Ministry spokesman.

The ISAF confirmed that two of its soldiers and four “ISAF-contracted civilians” were killed in the attack. It did not confirm if there were injured foreigners.

The nationality of the deceased were not known, but the US newspaper New York Times said they were US advisers.

Afghan President Hamid Karzai condemned the attack.

Thursday’s bombing made May the deadliest month this year in Kabul, where there has been a relative calm since March. It was also one of the deadliest incidents for foreign troops this year.

The explosion caused widespread damage, setting nearby houses on fire, police spokesman Hashmat Stanekzai said.

Local television footage showed a car burning alongside a destroyed armored vehicle amid scattered rubble and vehicle parts.

Some of the mud houses in the neighbourhood were damaged while windows were shattered and the street was left with a large crater.

The Afghan insurgent group Hezb-e-Islami-Hekmatyar claimed responsibility for the attack.

The group, not known to be affiliated with the Taliban, is an Islamic party opposed to NATO’s military mission in Afghanistan, which it called “a foreign occupation.” US soldiers travelling in the convoy were the target of the bombing, Haroon Zarghon, the movement’s spokesman, said by telephone from Pakistan.

In another suicide attack in March in Kabul, a bomber blew himself up at the gate of the Defence Ministry, killing nine people.

The incident occurred during a visit by US Defence Secretary Chuck Hagel, who was not in the area at the time.

The Afghan intelligence agency said it thwarts a number of attacks targeting Kabul every week. Large numbers of bombs and bomb-making materials have been confiscated by the agency this year, it said.

Thursday’s attack came two weeks after the Taliban declared the beginning of their spring offensive against Afghan and NATO security forces.

On Tuesday, a roadside bomb in the southern province of Kandahar killed four US soldiers while three Georgian soldiers were killed Monday in neighbouring Helmand province in a car bombing at the gate of their base.

The number of security incidents and civilian casualties in Afghanistan has risen in the first three months of this year, officials have said.

The UN Assistance Mission in Afghanistan said last month that civilian casualties increased almost 30 per cent in the first quarter in comparison with the same period last year while a report by the Afghanistan NGO Safety Office said attacks by the Taliban and other armed groups increased 47 per cent year-on-year in the first quarter.