The explosion just outside the gates of the provincial reconstruction team (PRT) in the relatively calm city of Herat was immediately claimed by insurgents loyal to the Taliban government toppled four years ago.
The powerful blast, which was heard throughout the city, sent shards of glass flying, shattered windows of nearby houses and cracked walls, witnesses said. It left a crater in the road, which was smeared with congealed blood.
"A suicide car bomb attack in front of the PRT office killed one Afghan guard and wounded three others," intelligence director Mohammad Musa Rasouli told AFP.
"The suicide attacker was driving a four-wheel-drive vehicle and detonated it in front of the PRT. It also damaged three other civilian cars nearby."
The NATO-led International Security Assistance Force (ISAF) confirmed the blast and said two Afghans were killed.
"Two local nationals died, another was seriously wounded. A civilian in the compound was lightly injured by flying glass," Captain Giorgio Buonaiuto said in the capital Kabul.
The compound houses 150 foreign personnel, most of them Italian troops, he said, correcting earlier reports that the base was manned by Spanish soldiers.
Police said a policeman had been killed and four others wounded.
A purported spokesman for the Taliban regime removed from power in 2001 said the movement was responsible for the attack.
"It was a suicide attack carried out by a citizen from Herat named Abdul Rahim," Yousuf Ahmadi told AFP by telephone. "The attack was aimed at foreign troops," he said.
Taliban militants have been waging an insurgency since their regime was toppled by US-led forces weeks after the September 11, 2001 attacks blamed on Al-Qaeda, which was sheltered by the Taliban government.
The fugitive head of the ousted regime, Mullah Mohammad Omar, last month promised a wave of suicide attacks as part of a spring offensive.
There has been a stream of such attacks in the past weeks, most of them in south and few killing anyone besides the attackers.
On Friday a suicide blast outside a base for British and US troops in the southern province of Helmand wounded two US soldiers and an American civilian.
The Taliban said it was also responsible for the blast outside the main gates of a PRT in the provincial capital Lashkar Gah.
Another suicide blast in the province's Sangin district on Friday appeared to have been targeted at Afghan troops. Only the attacker was killed.
There are around 30,000 troops from around 30 countries in Afghanistan helping the new government to extend its authority to allow rebuilding after 25 years of war that ended with the ouster of the Taliban.
ISAF, which has about 10,500 troops from more than 30 nations, is based mainly in Afghanistan's north and west, which have seen relatively little of the insurgency-linked violence plaguing the south and east.
However the force will deploy to the south in the coming months, and later to the east, eventually merging with a 20,000-strong US-led coalition that has been in these areas to hunt down militants.