Mystery and confusion surrounded a US cargo plane seized at Harare airport on suspicion of carrying mercenaries after Equatorial Guinea said today that it had arrested a 15-man advance party linked to the impounded aircraft.

The ageing Boeing 727-100, which is alleged by Zimbabwe to have been carrying 64 mercenaries of various nationalities, including South Africans, took off from Polokwane airport in Limpopo Province yesterday, before being held by Harare over a false declaration concerning its cargo.

Aviation authorities are still trying to establish the aircraft's destination. It has been variously reported to have been heading for Burundi, the Democratic Republic of the Congo and the oil rich West African nation of Equatorial Guinea.

But the Government of Equitorial Guinea, a tiny former Spanish colony wedged between Gabon and Cameroon, said that it had arrested 15 mercenaries suspected of plotting a coup.

"Some 15 mercenaries have been arrested here, connected with that plane in Zimbabwe. They were the advance party of that group," said Agustn Nse Nfumu, the Information Minister.

The 15 included a group of black and white South Africans, along with nationals from Germany, Kazakhstan and Armenia, Mr Nfumu added.

Beeld, the Afrikaans-language daily, also reported South African intelligence sources confirming that the aircraft was on its way to Equatorial Guinea, sub-Saharan Africa's third-largest oil producer.

Rumours of an impending coup were rife in the country amid growing tensions among President Mbasogo's family, whose members hold most of the top jobs in the country.

Witnesses who saw the aircraft being loaded before it left South Africa said it contained equipment such as hammers, bolt-cutters and shovels.

"It looked more like people going on a mining expedition," one witness said.

"It's certainly not the type of stuff I would like to start a war with," he added.

One unconfirmed report claimed that the aircraft was on its way to Bujumbura, packed with de-mining equipment, as part of an international effort to clear minefields in the Democratic Republic of Congo.