Incumbent president Karzai leads in presidential elections

The country has been in political limbo since last Thursday's election, with Karzai and chief rival Abdullah Abdullah each claiming victory. Abdullah and other challengers have accused Karzai and the authorities of widespread fraud.

The elections are a major test for Karzai after eight years in power and for U.S. President Barack Obama, who has poured in thousands of extra troops as part of his new regional strategy to defeat the Taliban and stabilize Afghanistan.

A relative lull in violence since the election was shattered later on Tuesday by a massive truck bomb in southern Kandahar city, the birth place of the Taliban, which killed 13 people and wounded over 30, hospital officials and police said.

A Reuters witness said the blast set fire to a restaurant and leveled a shop and three houses.

The blast also caused several cars to explode, leading one security source to initially say a series of simultaneous bombs had been detonated near buildings housing a Canadian development project, Afghan security forces and a Japanese contractor.

"The explosive materials were stored in a truck," said Kandahar deputy police chief Ahmed Shah.

There was no information immediately available on the nationality of the casualties. A Taliban spokesman claimed responsibility for the blast.