Official and aid workers said on Thursday that Dongiro warriors had crossed into Kenya last Friday and attacked Turkana herdsmen in order to steal their animals.
Njenga Miiri, the district commissioner for Turkana, said the fighting killed 30 of the raiders and eight Kenyans, all of them women and children.
The clashes took place in the remote village of Lokamarinyang, along the Kenya-Ethiopia border and 420km north of Lodwar, area aid worker Lucas Ariong said.
News of the battle only reached the regional capital on Thursday morning, nearly a week later.
Cattle rustling by nomadic tribes in the semi-arid region that encompasses northern Kenya, Uganda, southern Sudan and Ethiopia is common, and the tribes in the area do not respect national borders.
A drought that has left 11.5 million people in the area, most of them nomads, in need of water and livestock has exacerbated tensions between the tribes.
A drought has left local people in
need of water and liverstock
Miiri said the young warriors, who recently had undergone a ritual marking their transition into adulthood, were from the Naita area of southern Ethiopia and were carrying Kalashnikov assault rifles.
He said they were apparently trying to demonstrate their bravery by raiding a Turkana settlement to steal 300 cows and goats.
Turkana warriors pursued the Dongiro men and eventually caught up with them, killed 30 of the raiders and took back their livestock, Miiri said.
Kenyan police and soldiers have been dispatched to the area, he said.
Aid workers have expressed concern that as water sources dry up and livestock begin to die because of the drought, such attacks will become more common.