Brigadier-General Eric Tremblay, an Isaf spokesman, said the attack appeared to have been directed towards security forces and not the public.

"The vehicle was stopped, he could not move in, and he decided to detonate the vehicle with the subsequent explosion," he told Al Jazeera. 

Taliban 'responsible'

The Taliban quickly claimed responsibility for the blast.

"The suicide car bomber exploded near the Isaf headquarters and killed several foreign troops," Zabiullah Mujahid, a Taliban spokesman, told the Reuters news agency.

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"The target was the US embassy, but we could not reach it."

Al Jazeera's James Bays, reporting from Kabul, said: "This bomb has taken place in one of the areas that should be one of the most secure areas of the Afghan capital.

"It is a scene of devastation.

"It is clear the bomber managed to make it through the outer layer of security, which is provided by the Afghan security forces, and right outside the main gate of the military base."

The attack comes five days before Afghans head to the polls for presidential elections.

The Taliban's leadership has called Thursday's election an American invention, and has threatened to disrupt voting across the country.

Intimidation pervasive

According to the United Nations, violence and intimidation have already disrupted planning and campaigning in the country's south, and could prevent many Afghans from casting their ballots.
 
Election officials have said fears of violence could prevent hundreds of polling stations from opening and that voting was unlikely in nine of 365 districts.

In recent weeks, fighters have staged a number of attacks on provincial government buildings in the south, and also launching raids in the north and west.

Taliban fighters clashed with police in the northern province of Baghlan late on Friday, killing five people, including two children, police said.

Election race
 
Opinion polls have indicated that Hamid Karzai, the current Afghan president, leads the race, but without enough support to win an outright majority.
 
The US-funded International Republican Institute released poll results on Friday, suggesting Karzai would win 44 per cent of the vote while his main challenger Abdullah Abdullah, a former foreign minister, would win 26 per cent.
 
If no candidate wins more than 50 per cent in the first round, a runoff will be held between the top two.
 
In parallel with the presidential poll, Afghans will also vote for provincial councils.