The bomb blast occurred in Kabul's eastern district of Bagrami which is predominantly inhabited by Pashtuns from south and west Afghanistan. The district is said to harbour many Taliban supporters. The local chief of police confirmed the killings:

"This morning at 9.15, a remote-controlled mine exploded in the district of Bagrami," he said. "Two cars, in which there were foreigners, were damaged. Three German citizens were killed and one more is injured. The attackers are enemies of peace and of Afghanistan's security."

Officials in Kabul and Berlin confirmed that three German policemen had been killed -- all of them had diplomatic status.

Protecting embassy

Another policeman was also injured. The German Interior Minister Wolfgang Schaeuble said the three officers were deployed to Afghanistan to protect the German embassy and the ambassador Hans-Ulrich Seidt.

They were travelling in two white heavily-armoured four-wheel drives on their way to a training when the blast occurred.

Officials said the explosion, which was so powerful that it ripped through the armour plating and overturned one vehicle, was probably triggered remotely.

Taliban responsible

In a phone call to a news agency, the Taliban spokesman Yusuf Ahmadi claimed responsibility for the attack.

The Taliban also said on their web site they had destroyed a foreign military vehicle in a bombing in Bagrami. They claimed nine soldiers were killed, but they are known for exaggerating their casualty figures.

Over the past year, militant attacks have massively increased in number. Bomb blasts and suicide attacks tend to target Afghan and foreign security forces in particular.

German targets?

But more and more, foreign civilians are becoming the target, including Germans, who initially felt somewhat protected from such attacks as they were involved mainly in reconstruction efforts.

However, in March, a German aid worker was executed. And in May, three German soldiers were killed in a suicide attack in Kundus in the country's north.

Last month, a German engineer was killed by his kidnappers, and a second hostage is still being held captive in Ghasni in the south-east of Afghanistan.

It is still unclear as to whether Germans are being specifically targeted or whether they are falling victim to a general rise in attacks on foreigners.

There are some 3,000 German troops engaged in a reconstruction mission in northern Afghanistan within the NATO-led International Security Assistance Force, or ISAF.