Communal violence over water and pastureland in the Tana Delta left more than 100 people dead in recent weeks.
Victoria Nuland, a spokeswoman for the U.S. State Department, called on the Kenyan government to encourage rival factions to come together for peaceful dialogue.
"It is also important that those who have committed crimes are held accountable through transparent, fair and thorough investigations and trials," she said in a statement.
Kenyan President Mwai Kibaki sacked Dhadho Godhana, assistant minister for livestock, after he was arrested on charges of inciting violence in the Tana district, reports The Daily Nation newspaper in Kenya.
Human Rights Watch said more was needed from Kenyan authorities as other politicians and organizers are suspected of encouraging attacks in the Tana Delta.
"For decades the Kenya police have failed to investigate politicians who may be implicated in serious crimes," Leslie Lefkow, deputy Africa director at Human Rights Watch, said in a statement. "If they are ready to do so now the authorities must be even handed and investigate all sides."