At least seven passengers were injured when a double-decker bus exploded in southern Philippines on Monday.

The commuters were inside a Husky Tours bus parked at the Isulan bus terminal in Sultan Kudarat province when an improvised explosive was detonated on April 17.

The blast wounded seven people - including three children and two elderly people - who were rushed to the provincial hospital. They are now in safe condition after being treated.

Police said the bomb had been placed in the middle of the lower level of the two-decker. It exploded at around noon while more passengers were boarding the bus at the terminal.

Isulan police chief Lieutenant Colonel Richelu Alucilja said investigators have already identified three 'persons of interest' in the case.

He added: 'A second unexploded IED was discovered by a bomb disposal team who combed the bus after the explosion. It was detonated by bomb experts.'

Sultan Kudarat police assistant director for operations Lieutenant Colonel Lino Capellan said a special task force has been created to probe the blast.

Police are on alert as investigations continue, he said.