Afghan policemen and onlookers gather at the site where a car bomb exploded in the centre of the southern Afghan city of Kandahar. (Ahmad Nadeem/Reuters)

A suicide bomber targeting a compound shared by foreign companies set off a massive explosion late Thursday in the southern city of Kandahar, blowing out windows across the city and killing at least six people, the president's half-brother said.

The blast came hours after another car bomb exploded outside a Kandahar hotel and injured at least eight people.

Fighting in the north of the country, meanwhile, left four German soldiers dead, officials said, while insurgents carjacked UN vehicles elsewhere in northern Afghanistan.

The Thursday night explosion occurred when the suicide bomber managed to get his car past one barrier leading into a compound shared by a number of western companies, then set off the explosion at a second barrier, said Ahmed Wali Karzai, the half-brother of Afghan President Hamid Karzai and an influential figure in the city.

He said at least six people were killed, including three foreigners and three Afghan soldiers, and several more people had been injured, Ahmed Wali Karzai said.

Authorities, including NATO forces and Afghan soldiers, police and intelligence officers, swarmed the scene soon after and sealed off the roads.

The blast blew out windows as far as four kilometres away. The compound includes the offices of the international contracting company Louis Berger Group, the Afghanistan Stabilization Initiative and the aid contracting company Chemonics International.

Little security

Kandahar, the largest city in the southern Taliban heartland, has been shaken repeatedly by attacks in recent weeks.

On March 13, a suicide squad detonated bombs at a newly fortified prison, police headquarters and two other locations, killing at least 30 people.

An earlier explosion, in front of the Noor Jehan Hotel, shattered windows in the shabby four-storey structure, destroyed five vehicles and damaged a number of shops in the area. At least two of those injured were in serious condition, said local official Nidah Mohammed.

The hotel, located in a busy downtown commercial district, is home to a number of foreign news organizations and has little security.

International forces present in the city rarely patrol through the area. The news organizations are largely staffed by Afghans, and there was no immediate indication foreigners were among those hurt.

Four German soldiers were killed and five wounded in fighting Thursday in Baghlan province, according to the Defence Ministry in Berlin. It said fighting broke out after a German Eagle armoured vehicle was struck by what was believed to be a rocket around noon.