Zimbabwe's prosecutors has surprised 70 men arrested last week over an alleged coup plot in Equatorial Guinea by formally presenting them with a new charge, that of plotting to murder the president of the country, Teodoro Mbasogo.

The men already face charges of attempting to buy arms illegally and all but three face a further charge of breaking immigration regulations.

Lawyers have said the government had been searching for legislation imposing more severe punishment against the men.

"They were given another charge today," said Jonathan Samkange, their defence lawyer, after he left Chikurubi Prison Harare on Harare's outskirts, where the group is being held.

"They are accused of conspiring to murder the president of Equatorial Guinea.

"The police and the attorney-general say they will be brought to court tomorrow (Wednesday) at 10am," he added.

The governments of Zimbabwe and Equatorial Guinea governments last week reported arrests here and in Malabo, the capital, of different groups they said were co-ordinating a plot to oust the tiny oil-rich West African state's 62-year-old leader.

Authorities in Harare said that the Boeing had 727 flown from South Africa with 64 alleged mercenaries and three crew members to Harare, where they were to be met by an advance party of three who were to have purchased a range of AK47 rifles, light machine guns, pistols, mortars, rocket launchers and grenades meant to be used in a coup in the former Spanish colony.

In what government officials said was a joint intelligence operation involving the governments of South Africa, Zimbabwe and Equatorial Guinea, the plane was impounded as soon as it landed at Harare international airport and the 67 passengers and crew were arrested, as were an advance party.

The leader of the advance group was named as Simon Mann, an old Etonian and former officer of the British Special Air Services Regiment and said to be the founder of the now defunct South African mercenary recruitment organisation, Executive Outcomes, and connected to its British-based successor, Sandline International.

"All 70 have denied the allegation," Samkange said. "It's nonsensical. They are going to have big problems with the jurisdiction of the court over an offence that has may have taken place in several countries".

Last week foreign minister Stanislaus Mudenge said the group would face "the severest punishment in our statute books, including capital punishment," but up until Tuesday, acting attorney-general Bharat Patel was saying the only legislation in Zimbabwean law that could be used were immigration regulations and firearms control legislation, regarded much lesser offences.

A conviction for conspiracy to murder in a plot that had not been carried out and in which no-one had been hurt could not possibly carry the death sentence, senior lawyers said.

"It could result in a fairly long prison term," said one lawyer.

A South African, Nic du Toit, has appeared on Guinean state television where he "confessed" to being a conspirator in the alleged coup plot.

Zimbabwe government ministers have claimed that the alleged attempt was backed by MI6, the British espionage agency, the US' Central Intelligence Agency and the Spanish secret service. All have vigorously denied the claim.

The government says the men arrested on the aircraft were South Africans, Angolans, Namibians, Democratic Republic of Congo nationals and one Zimbabwean. Their lawyers say they all carry South African passports. – Sapa.