NAIROBI, Kenya (AP) - There's been an explosion on a bus in Kenya's capital.

Police and the Red Cross say at least five people were killed and more than two dozen injured.

Kenya has been hit by a string of grenade attacks in recent months blamed on sympathizers of al-Shabab, Somalia's Islamist extremist rebels 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.

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