KABUL - A BOMB ripped through a vehicle in a US-led military convoy outside the Afghan capital on Wednesday causing casualties, the military and witnesses said.

The vehicle was destroyed in the attack, about 35 kilometres from Kabul, where foreign troops cordoned off the area as an ambulance helicopter swooped in to pick up casualties, an AFP reporter said.

'A military vehicle was struck by an IED (improvised explosive device),' US Lieutenant Colonel Christian Kubik confirmed to AFP. He could not immediately comment on casualties.

An Afghan driver on the busy road, which connects the capital and the largest US base in Afghanistan at Bagram, said at least one foreigner died.

'I saw one of the foreigners who was stuck in the wreckage of the vehicle and he was dead. They were trying to pull him out,' said the driver, Sakhi Dad.

'It was a remote-controlled mine, I think, because all I saw was a blast and then I pushed the brake and stopped,' he told AFP.

The road is regularly used by the US military, which has at least 38,000 soldiers in Afghanistan to help the government fight a Taleban-led insurgency and rebuild its security forces, which were destroyed by years of war.

The extremist Taleban vowed last month to step up attacks on foreign soldiers as well as Afghan troops and officials.