An explosion has struck a bus carrying members of the presidential guard in the Tunisian capital, killing at least 11 people.

The blast occurred on the tree-lined avenue in the centre of Tunis, the interior ministry said.

It was unclear what had caused the explosion, which came 10 days after authorities increased the security level in the capital and deployed security forces in unusually high numbers.

Earlier this month, Tunisian authorities announced the dismantling of a cell it said had planned attacks at police stations and hotels in the seaside city of Sousse, about 150 kilometres (95 miles) south-east of Tunis.

A luxury beach hotel in Sousse was hit by an attack last June that left 38 people dead. In March, an attack at Tunisia's famed Bardo museum near the capital killed 22 people.