More than 60 people were also hurt as buildings collapsed in the attack in the centre of the city, doctors said.

The explosion took place shortly after the first results were announced in the presidential election.

No-one has yet claimed responsibility for the attack in Kandahar, which is a stronghold of the Taliban. Some reports said bombs were packed in cars that exploded simultaneously, others that a truck or tanker was used.

The explosion struck close to the offices of the Kandahar provincial council, trapping people under rubble.

Earlier a bomb killed four US soldiers, making 2009 the deadliest year for foreign troops since the US-led invasion overthrew the Taliban.

A British soldier also died from wounds suffered in an explosion near Sangin in Helmand province on 15 August, the Ministry of defence said.