Suspected al shabaab
Suicide bomb in market in Somalia capital kills 39
Al Shabaab, the Islamist insurgent group that is fighting the UN -backed Somali government, did not immediately claim responsibility.
The attack came at 9:30 am as the buses were travelling in Mandera county to Mandera town from the capital.
The Islamist militants say they attacked an AMISOM base manned by Ethiopian troops.
The Pentagon said the air strike took place in the vicinity of Saakow, Somalia.
The attack highlights the challenge of halting guerrilla-style raids by the group.
The group said its mujahideen fighters had killed 40 people despite initial reports of 36.
The group said Kenya received repeated warnings to pull its troops out of Somalia.