Abdul Ahad, the Gereshk district governor, said that at least 21 people were wounded in the blast.

The base is home to members of the Nato-led International Security Assistance Force (Isaf), mainly from the United States and Britain.

Convoys delivering fuel and other supplies to the almost 90,000 foreign troops in Afghanistan have previously come under attack in Afghanistan and neighbouring Pakistan, but there have been few incidents in recent months.

The attack came as the US prepares to boost the separate US force by more than 30,000 troops.

Washington moved to reassure its Nato allies on Friday that Isaf was still a major part of the battle against the Taliban.

"I would not minimise the importance of 32,000 Nato and partnering troops in Afghanistan and the role that they and their civilians and their development people are playing in virtually every part of the country," Robert Gates, the US defence secretary, said after a meeting of Nato defence ministers.

At the meeting in Brussels, the ministers agreed to a new command structure for the force, which will increase the influence of US generals by creating an extra headquarters to oversee day-to-day operations.