By Cris Chinaka and David Clarke

Harare - Zimbabwe threatened on Wednesday to execute 60 suspected mercenaries detained this week and accused American, British and Spanish spy agencies of involvement in a plot to topple Equatorial Guinea's government.

Equatorial Guinea, which has arrested what it called an advance party of 15 mercenaries, said "enemy powers" and multinational companies had been plotting against the small oil-producing Central African state.

The two countries, 2 000km apart, have put their security forces on high alert since Zimbabwe detained a Boeing 727 carrying 60 men, most of them South Africans, Angolans and Namibians, both white and black, on Sunday.

Associates of the men say they are innocent mine guards swept up in a bizarre misunderstanding.

"They are going to face the severest punishment available in our statutes, including capital punishment. We will give them all the rights they are entitled to," Zimbabwean Foreign Minister Stan Mudenge told a news briefing.

"They were aided by the British secret service, that is MI6, (the) American Central Intelligence Agency and the Spanish secret service," Zimbabwe's Home Affairs Minister Kembo Mohadi told a news conference.

Mohadi, whose country has been bitterly at odds in recent years with Washington and former European colonial powers, said Equatorial Guinea's police and army heads had gone along with the plot against President Teodoro Obiang Nguema Mbasogo.

The CIA declined to comment on Zimbabwe's spying charge, but American officials denied the allegation.

"There was no US interest or involvement in such a plot," said one US official, speaking on condition of anonymity.

Spain also denied involvement in any plot in the former Spanish colony.

A British Foreign Office spokesperson said: "I have no information on whether any security services were involved at all. But we certainly wouldn't comment on our security services anyway."