A suicide car bomber in Kandahar wounded nine people [EPA]

At least 47 people have been killed, more than a dozen of them civilians, in violence across Afghanistan ahead of a planned US review of its strategy in the near decade-long conflict.

In one incident on Saturday, a NATO air raid killed at least 25 suspected fighters in eastern Afghanistan.

Officials said a NATO force out on patrol had called in air support after encountering an "imminent threat" from fighters in the Nari district of Kunar province, which has been the scene of heavy fighting along the Pakistani border.

Elsewhere, in southern Afghanistan, a roadside bomb killed 15 civilians and a car bomb wounded five police.

In Kandahar, a suicide bomber driving a police car wounded nine people, when the car exploded near a police headquarters.

The blast blew out the windows of buildings up to a kilometre and a half away, officials said.

Street protest

In the east, seven men died in a disputed incident that sent hundreds pouring onto the streets of Gardez city in a protest that turned violent.

Armed police and protesters fired at each other and burning tyre barricades filled the streets with smoke. Six civilians and two policemen were wounded, said Nader Noori, doctor at the Gardez hospital.

Heavy fighting continues in the east even though the main focus of the war is in the south where NATO forces have pushed deeper into Taliban strongholds in Helmand and Kandahar provinces.

The violence comes as Barack Obama, the US president, prepares to unveil a review of his Afghanistan war strategy next week, although officials have said they do not expect it to result in any major policy shifts.

Obama has pledged to start bringing home US troops from July 2011, but has not yet decided on the pace or scale of that withdrawal.

Any draw down is likely to be more symbolic than substantial, but his 2011 target has drawn criticism from some Republicans who say announcing a date emboldens the Taliban.