The White House condemned "in the strongest terms" a tribal attack in southeastern Kenya that left 45 dead, amid fears of growing tensions between rival communities.
Villagers were hacked to death and their homes were torched in Friday's attack on the village of Kipao in the Tana delta region, an area where deadly tribal violence killed another 100 people earlier this year.
The victims included women and children.
"The United States condemns in the strongest terms the renewed violence in Kenya's Tana River region," White House spokesman Jay Carney said in a statement.
"This latest incident represents a disturbing escalation of the tragic violence witnessed by these communities in August and September."
Police attributed the killings to a disarmament operation that stoked long-simmering tensions between the rival Orma and Pokomo communities in the Tana river delta area.
Relations have long been fraught between the two communities, with conflicts flaring intermittently over access to land and water points.
The White House called on the Kenyan government, police and Orma and Pokomo leaders to "bring an end to this deadly cycle of conflict, intensify efforts to establish a durable peace in the Tana delta and hold to account the perpetrators of these heinous acts."
But the violence could also be linked to a March election, the first since Kenya was gripped by deadly violence after a December 2007 vote.
"With historic elections approaching in March, peace and stability are essential to Kenya's continued progress," Carney said.
"The United States urges the people of Kenya to assert their rights through peaceful means, as provided for in Kenya's constitution, so that all Kenyans may realize a secure, democratic and prosperous future."
In Kenya, regional police chief Aggrey Adoli said the number of dead had risen to 45 from an earlier toll of 39.
Another police source said on condition of anonymity that the dead included 16 children, five women and 10 men, along with 14 assailants.