A spokesman for the Islamists said the attack was in revenge for raids that Kenyan security forces carried out over the past week on mosques in the port city of Mombasa.
"The Mujahedeen successfully carried out an operation near Mandera early this morning, which resulted in the perishing of 28 crusaders, as a revenge for the crimes committed by the Kenyan crusaders against our Muslim brethren in Mombasa," Ali Mohamud Rage said in a statement sent to AFP.
The bus attack came after a week that saw one person shot dead and more than 350 people arrested as Kenyan security forces carried out raids on mosques in the port city of Mombasa, looking for arms and Shebab sympathisers.
The raids have raised tensions in a city already hit by a string of bombings and shootings and police refusal to take off their shoes while entering mosques had angered many.