The crash occurred in a forest around 8 a.m. and killed Internal Security Minister George Saitoti, his aide, two bodyguards and two pilots, the Kenya Broadcasting Corp. reported.
The minister was on his way to a church fundraising event in Ndhiwa, the Sunday Nation newspaper in Nairobi said.
Rose Kwamboka told the newspaper she watched the helicopter's final seconds.
"The chopper was filled with smoke before it crashed," she said. "I heard two explosions as it went up in flames."
Political campaigning for next year's presidential election is in high gear in Kenya, although all candidates canceled their appearances when news of the crash broke, the KBC said.
President Mwai Kibaki issued a statement of condolence to the men's families.
"The deaths of the six Kenyans is a devastating loss to our country," he said.