"We were having breakfast when we heard a huge bang. All the glass was shattered," Abdul Jamil, a worker at a petrol station across the street from the site, said.
A damaged white civilian vehicle could be seen at the debris-strewn blast site on a main road about 100 metres from Camp Phoenix on Jalalabad road, the main road leading eastwards out of Kabul.
Witnesses said a second civilian vehicle destroyed in the blast had been removed, while US troops in armoured vehicles blocked off the site.
Nato, together with the US military, has more than 100,000 troops in Afghanistan.
Camp Phoenix, on the outskirts of Kabul, is run by US forces, with some Nato member nations maintaining a presence there.
It is also a base for the Afghan army, which is being trained by international forces in the hope it can take over the responsibility for fighting the Taliban.
The camp occasionally comes under attack, mostly from rocket and mortar fire, though without casualties.
Gordon Brown, the UK prime minister, said on Friday that Britain was trying to persuade its military allies fighting the Taliban in Afghanistan to send another 5,000 troops.
Brown said he had begun a diplomatic effort to encourage members of the Nato-led International Security Assistance Force (Isaf) to share the burden of the combat and help train Afghan soldiers.