A local police official says a group of ethnic Pokomo, armed with spears and firearms, raided a village belonging to the Orma people in the pre-dawn hours of Friday.
The Kenya Red Cross says 13 children and six women were among those killed in the attack on Kipao village. It says at least 45 houses were set on fire.
Police say they have identified where the attackers came from and are pursuing them.
The two groups have waged a series of deadly attacks and counter-attacks as part of an ongoing dispute over land and water.
In August and September, more than 100 people were killed during clashes between the Pokomo, which is largely a farming community, and the Orma, which consists of mainly herders.
The United Nations says some of the killings may be related to increased tensions surrounding next year's general elections.
More than 1,100 people were killed in post-election violence after Kenya's 2007 polls. VoA.