"Police on the scene have found six persons shot dead," said police chief Joseph Boinnet. "The hunt for the terrorists is under way."
Boinnet said two people were also injured. It was unclear whether the casualties were civilians or police officer guards aboard the buses.
The early morning ambush happened near El Wak in Mandera county, an impoverished rural part of the country where similar deadly attacks in the past have been claimed by the Somali-led Al-Qaeda group, the Shabaab.
A similar raid in December 2015 left two people dead, after Muslim passengers shielded their Christian fellows.