Police officials say al-Shabab militants from Somalia have hijacked a bus in northern Kenya and killed 28 non-Muslims on board.

The two police officers said Saturday that the bus traveling to the capital Nairobi was hijacked 50 kilometers (31 miles) from Mandera town. The officers say the gunmen singled out non-Muslims and shot them dead.

The officers insisted on anonymity out of fear of reprisals because of an order from Kenya's police chief that officers not speak to the media.

Kenya has been hit by a series of gun and bomb attacks blamed on Somalia's al-Qaida-linked al-Shabab militants since it sent troops into Somalia in October 2011. Authorities say there have been at least 135 attacks since then, including the Westgate Mall attack in which 67 people were killed.