Rhodesia said today it is using U.S. designed Bell helicopters in the war against black nationalist guerrillas, despite trade sanctions that prohibit the sale of such items to the white minority government.

"The U.S. Bell 205 helicopter is now in service with the Rhodesian air force," a military spokesman said, confirming reports that up to 11 civilian versons of the Huey choppers used in Vietnam were in Rhodesia.

Rhodesian spokesmen declined to say how the American helicopters were obtained in violation of U.N. trade sanctions against Rhodesia.

In Fort Worth, Tex., a spokesman for Bell Helicopter said that according to company records, "no Bell helicopters has ever been sold to anyone in Rhodesia."

"Bell Helicopter Textron knows nothing regarding the report that 11 Bell helicopters have been smuggled into Rhodesia," the spokesman siad.

"Every Bell helicopter exported outside the U.S. is exported in accordance with regulations of the U.S. Department of Commerce and the U.S. Department of State," the spokesman said. "Bell's licensees are bound by the same regulations."

U.S. officials were reported attempting to determine the source of the 127 mph craft, which, in its civilian form, can carry up to 15 passengers.

In Washington, the State Department said the government was investigating.

Meanwhile, two Rhodesian finance ministers flew to neighboring South Africa, apparently to discuss Rhodesia's fuel supplies that were sharply reduced this week black nationalist guerrillas blew up a major fuel dump.