Officials in South Africa and Zimbabwe have echoed allegations by Equatorial Guinea that foreign mercenaries plotted to overthrow its president.
President Obiang says "enemy powers" are trying to overthrow him
Zimbabwe has linked the alleged plot to 64 men detained in Harare after their plane was grounded on Sunday.
Zimbabwe claimed US, British and Spanish spy agencies were helping the "mercenaries" in the plot.
Equatorial Guinea's President Teodoro Obiang Nguema also said multinational firms were involved in the conspiracy.
According to Zimbabwe's Home Affairs Minister Kembo Mohadi, the "mercenaries" were "aided by the British secret service, that is MI6... American Central Intelligence Agency and the Spanish secret service".
The BBC's Alastair Leithead in South Africa describes the British and American links as "spurious", but adds there is growing evidence that the mystery does relate to a coup attempt gone wrong.
South African government officials said they believed the suspected mercenaries had been en route to Equatorial Guinea to help carry out a coup, and had stopped at Harare to pick up weapons.
South African Foreign Minister Nkosazana Dlamini-Zuma said the government was trying to establish what was going on, but added that "indeed there was a link between the plane and Equatorial Guinea".
Earlier, the Equatorial Guinea president said a group of 15 mercenaries had been arrested in the country on suspicion of trying to overthrow him.
He said they were linked to those held in Zimbabwe.
On state radio, Mr Obiang said South Africa had warned him that a group of mercenaries was heading for his country.
He also accused multinational companies of being behind the plot to overthrow him, and said any firms that financed such operations would be classified as enemies.
The 64 suspected mercenaries are currently imprisoned in Zimbabwe and are expected to appear in court on Thursday.
The Zimbabwean authorities have warned they could face the death penalty if found guilty.
Military equipment was reportedly found on the impounded plane
The men - said to be Angolans, South Africans and Namibians - were detained after their plane was impounded on Sunday evening at Harare International Airport.
A senior executive from the company that chartered the plane said they were going to Burundi and the Democratic Republic of Congo to work as security staff at the mines.
He said they stopped in Harare to pick up cheap mining-related supplies.