The attack in Khost province late Wednesday was the latest in a spate of more than 20 such blasts in the past four months, pointing to a change of tactics in an insurgency that erupted after the 2001 fall of the Taleban militia.

Three soldiers were also wounded after the attacker drove the vehicle into the Afghan army convoy in Khost’s Bak district and set off the deadly blast, defence ministry spokesman General Mohammad Zahir Azimi said.

“Three soldiers and one civilian were martyred and three soldiers were wounded in the car bomb suicide attack,” he said.

The soldiers had been providing security for a road construction project, a local police officer said on condition of anonymity.

The deadliest suicide attack in Afghanistan since the start of the insurgency killed more than 22 people in the southern town of Spin Boldak on January 16.

It followed a spate of such blasts -- previously rare in this country -- targeted mainly at Afghan security forces and the nearly 30,000 foreign troops who have been helping to stabilise Afghanistan since the Taleban were removed.

The Taleban were ousted in a US-led attack in late 2001 after they failed to surrender Osama bin Laden following the September 11 attacks on New York and Washington.