David Chater, Al Jazeera's correspondent in Kabul, said the attack was immediately outside camp Phoenix, a US base, on the main Kabul-Jalalabad highway.

"There are huge numbers of security forces around the area at the moment," Chater said.

"The biggest danger is that there might be a second device in the area after they've [it was claimed by the Taliban] attracted the security forces to the scene.

"This is the most dangerous road in Afghanistan, the highway is used by so many military patrols. It is also the main route for the resupply of Nato and American troops here," he said.

Nato's international security assistance force (ISAF) said "Initial reports indicate the cause of the explosion was a vehicle-borne IED," referring to an improvised explosive device in a car.

Pakistan links

The bombing comes on the same day Afghan officials said an attack in Kabul on January 18 was carried out by fighters smuggled over the border from Pakistan.

Officials released video footage of a man arrested in connection with the attacks who said that the Haqqani network, a group of Afghan fighters based in Pakistan, were behind the offensive on civilian and government buildings near the presidential palace.

The attacks, said to be the most co-ordinated offensive on the capital since the US-led invasion in 2001, took place while Hamid Karzai, the Afghan president, was swearing in some of his cabinet ministers.

At least five people were killed and about 38 more wounded in the protracted gun battles that followed.