NAIROBI, Kenya — A Kenya police official says that an explosion on a bus in Kenya's capital has killed at least five people and injured 13.

Nairobi police chief Moses Ombati says at least five people were killed Sunday after an explosion on a 25-seater public transportation vehicle, and that there will likely be more casualties. He says 10 men and three women were injured in the blast. He did not give details on the cause of the blast.

Kenya has been hit by a string of grenade attacks that are blamed on sympathizers of al-Shabab, Somalia's Islamist extremist rebels who are linked to al-Qaida.

Al-Shabab has vowed to carry out attacks on Kenya because it sent troops into Somalia last year to fight the rebels, who are considered a threat to Kenya's security because they have been blamed for kidnapping foreign tourists and aid workers in Kenya.