At least 13 people have been killed in a powerful bomb explosion in the southern Afghanistan city of Kandahar, reports quote officials as saying.

The bomb exploded outside the headquarters of the provincial police.

Police chief Bashir Khan told the Associated Press that the bomb may have been "attached" to a motorcycle parked outside the building.

Last week, fierce fighting between troops and Taliban fighters left at least 25 dead in southern Afghanistan.

The upsurge in violence comes as the US plans to hand more responsibility for security in southern Afghanistan to Nato allies.

Last year, attacks mainly in southern and eastern Afghanistan left more than 1,400 people dead. It was the country's bloodiest year since US-led forces ousted the Taliban in late 2001.

PHOTO CAPTION