Mandera County Commissioner Fredrick Shisia told Reuters the attack happened at 9:30 a.m. (0630 GMT) as the buses were travelling to Mandera town from the capital Nairobi.
"So far we are talking of six dead," he said, noting that the attack had happened on the road between Wargadud and Elwak.
The United States on Thursday warned its citizens against travelling to areas near the border with Somalia because of threats from Somali militant group al Shabaab.
Shisia said it was not yet clear who was behind the attack. Mandera has been the scene of frequent al Shabaab attacks in which dozens of civilians and security personnel have been killed.
The shattered windows of a bus ambushed by al Shabaab militants on its way to Mandera. Picture courtesy: The Star, Kenya
At least six people were killed when gunmen sprayed two buses with bullets on Friday in Kenya's Mandera county on the border with Somalia, a regional official said.