PESHAWAR, PAKISTAN - A bomb exploded at a mosque in northwest Pakistan during the main Muslim weekly prayers on Friday, killing at least four people and wounding 30 more, police said.

The explosion took place on the hilly outskirts of Dir Bala town in Upper Dir, a district bordering a region where the military has been pressing an offensive against Taliban militants for six weeks.

'There are four killed and 30 injured in a bomb blast inside a mosque during Friday prayers,' Ijaz Khan, the police chief in Dir Bala, told AFP.

The police official said he expected the casualty numbers to go up.