A cargo plane impounded in Zimbabwe on suspicion of carrying 64 mercenaries is believed to have departed from South Africa and may have had South African nationals on board, authorities said Tuesday.

Preliminary investigations indicated the Boeing 727-100 (search), which Zimbabwe authorities say is registered in the United States, took off from Wonderboom airport (search), north of the capital Pretoria, said Moses Seate, spokesman for the South African Civil Aviation Authorities.

It then continued north to Polokwane and on to Zimbabwe, where it was detained Sunday night at Harare International Airport, Seate said.

Aviation authorities are still trying to confirm the plane's owners, contents and intended destination, he said.

[Reuters reported Tuesday morning that 15 men had been arrested in Equatorial Guinea in connection with the plane seized in Zimbabwe. 

"Some 15 mercenaries have been arrested here in Equatorial Guinea and it was connected with that plane in Zimbabwe," Information Minister Agustin Nse Nfumu told Reuters. "They were the advance party of that group."]

South Africa's high commissioner to Zimbabwe, Jerry Ndou, was attempting to verify reports that some of the people on board were South Africans, said Foreign Affairs Ministry spokesman Ronnie Mamoepa.

Zimbabwe Home Affairs Minister Kembo Mohadi said Monday that authorities had impounded a U.S-registered aircraft carrying military equipment and 64 suspected mercenaries of varying nationalities.

State-run TV broadcast footage of a white plane with a blue stripe containing satellite telephones, radios, backpacks, sleeping bags, hiking boots, an inflatable raft, bolt cutters and what appeared to be a can of Mace. No weapons were shown.

The plane and its passengers, most of them whites, were taken to a nearby military airfield, the station said.

In Washington, State Department spokesman Richard Boucher said Monday there was no indication the aircraft was connected to the U.S. government.

Another Washington official, speaking on condition of anonymity, said it had been established that passengers and crew were all African and that no Americans were aboard the plane.

The plane's registration number, N4610, is assigned to Dodson Aviation Inc. of Ottawa, Kan. However, company director Robert Dodson said it had sold the aircraft about a week ago to a South African company, Logo Ltd. (search) That company could not immediately be traced.

President Robert Mugabe has repeatedly accused the United States and Britain of plotting to overthrow his autocratic regime.

Zimbabwe faces its worst political and economic crisis since independence from Britain in 1980. The government's often-violent seizure of thousands of white-owned farms for redistribution to blacks has plunged the country into turmoil.