The death toll in an explosion targeting Tunisia's presidential guard has risen to 13 and the perpetrators remain unidentified, the interior ministry said.

Another body was discovered in the charred remains of a bus carrying presidential guards that exploded in central Tunis on Tuesday, spokesman Walid Louguini said. The body has not been identified yet.

Mr Louguini could not confirm or deny Tunisian media reports that the body could be that of a suicide bomber who jumped on the bus just before the bomb exploded.

Troops fanned out around the capital and the president declared a month-long state of emergency after Tuesday's attack.

The blast rattled the country after two major attacks by Islamic extremists that targeted tourist sites earlier this year.