The grenade and gun attack was launched on a residential block in Mandera town when people were sleeping, police said.
It was the latest in a spate of deadly attacks targeting Christians in the mainly Muslim region.
In December 2014, al-Shabab killed 38 non-Muslims at a quarry after separating them from Muslim workers.
A few months earlier, 28 people were killed after Muslim passengers were split up from the other passengers.
The attack happened as "planned" and was aimed at Christians in Mandera town, a radio station run by al-Shabab has reported.
