Thousands of people have fled violence in Kenya's Rift Valley which has left at least 15 people dead.
The two groups have clashed for many years
More police have been sent to the area north-west of the capital, Nairobi, to control the latest clashes between Kenyan farmers and cattle owners.
Youths from Kikuyu and Maasai groups fought over the weekend using machetes, spears, bows and arrows and clubs.
Several huts were torched in the violence.
The trouble is thought to have started when Maasai herdsmen accused a local Kikuyu politician of diverting a river to irrigate his farm, prompting a water shortage further downstream.
With water being denied to their livestock, the Maasai are then thought to have damaged his pipes.
"The government is doing all it can to control the situation, I urge the combatants to lay down their weapons and talk peace," John Kamau, a senior local official, told Reuters news agency in the trading post of Mai Mahiu, where many Kikuyu have fled.
A large number of Maasai are reported to have fled their homes for Narok, further west.
The Maasai and Kikuyu communities have fought over access to water and grazing land since the 1960s.
Fighting last week involving the Maasai near the Maasai Mara game reserve displaced more than 2,000 villagers.
Kenyan President Mwai Kibaki is this week due to visit another area, Mandera, which has been the scene of similar violence, over the control of water, between rival Kenyan Somali communities.