Somalia-based al-Shabab militants have executed 28 people in an ambush on a bus in northeastern Kenya, policemen say.

Speaking on condition of anonymity, two police officers said that the bus, which was traveling to the Kenyan capital, Nairobi, with 60 passengers on board, was hijacked on Saturday some 50 kilometers (31 miles) from the town of Mandera near the Somali border.

Among those killed were public servants who were heading to the capital for the Christmas holidays, the officers added.

“Bandits ambushed a bus from Mandera that was heading to Nairobi at dawn and killed 28 passengers of the 60 that were in the bus," Kenya’s Ministry of Interior and Coordination of National Government said on its Twitter account.

Kenya currently has over 3,000 soldiers stationed in southern Somalia, where they have been battling al-Shabab. The country sent troops into Somalia in late 2011, after the militant group carried out a series of raids inside Kenya.

The al-Shabab militants have been pushed out of the Somali capital, Mogadishu, and other major cities in the country by the African Union Mission in Somalia, which is made up of troops from Ethiopia, Uganda, Burundi, Djibouti, Sierra Leone and Kenya.