By Agence France-Presse
The accident occurred in the Central Island National Park in Lake Turkana at around 8pm (1700 GMT) on Sunday, police said in a statement.
All five on board the chopper were killed.
"Security personnel dispatched to the scene confirmed that among the five were four Americans and the Kenyan pilot."
The cause of the crash was not immediately known, and the identities of the victims would be released once their next of kin have been informed, the statement said.
Known as the Jade Sea, Lake Turkana, which is popular with tourists, is the most saline lake in East Africa and the largest desert lake in the world.
It is also a candidate site for the birthplace of mankind.
Its islands are breeding grounds for the Nile crocodile, hippopotamus and several snake species while the lake itself is important to migratory birds.
Last year, UNESCO placed it on its list of endangered World Heritage Sites, in part because it is threatened by Ethiopia's construction of a hydroelectric dam on the Omo River, which replenishes the lake seasonally.