The three people indicted were named as Claude Chi, 40, of Lee's Summit, Missouri; Francis Chenyi, 49, of St. Paul, Minnesota; and Lah Nestor Langmi, 46, of Buffalo, New York, in a justice department statement released on Monday.
They allegedly solicited and raised funds for supplies, weapons and explosive materials to be used in attacks against Cameroonian government personnel and security forces, it said.
"In addition to more than $350,000 the defendants raised through voluntary donations, the indictment alleges Chi, Chenyi and Langmi conspired with others to kidnap civilians in Cameroon and hold them for ransom," the statement said.
"In some instances, U.S. citizens were extorted for ransom payments to secure the release of their kidnapped relatives living in Cameroon," it added. The ransom payments were then transferred to separatist fighters to fund their operations.
The three defendants each held senior positions in an organization that supported a group known as the Ambazonian Restoration Forces and other separatist fighters in Cameroon's Northwest region, and had been raising funds for them since 2018, the statement said.
Reuters was unable to immediately establish contact with the Ambazonian Restoration Forces or representatives for the defendants.