"The United States government has protested to the government of Zimbabwe concerning the outlandish and inaccurate allegations made publicly, or released to the press, about US involvement in a purported mercenary operation," said US embassy spokesman Samuel Kaerezi.
"The US government has no connection with either the individuals detained or the aircraft seized by Zimbabwean authorities," he said in a statement in Harare.
"No American citizens numbered among those on board the plane. If the government of Zimbabwe has any information to exchange, the US embassy is available to discuss," Kaerezi said.
US Secretary of State Colin Powell confirmed on Wednesday that, according to the available information, Zimbabwe was not the plane's intended destination, but he strenuously denied any US involvement in the affair.
"No American citizens numbered among those on board the plane"
spokesman,US embassy, Harare
Spain also dismissed the accusations as "absolutely false", with the foreign ministry vowing to complain formally to the authorities in Harare.
Zimbabwe's Home Affairs Minister Kembo Mohadi said at a news conference on Wednesday that one of the men, named as Simon Mann, fingered London, Washington and Madrid as their backers after Zimbabwe detained an aircraft with 64 suspected mercenaries aboard, plus three men who came to meet them.
The Zimbabwean government has said these men were heading to Equatorial Guinea, an oil-rich tropical state partly on the African mainland and part islands in the Gulf of Guinea, to join 15 other coup plotters who were arrested there earlier.
According to Mohadi, one detainee "has revealed that they (the mercenaries) were aided by the British secret service (MI6), the American Central Intelligence Agency (CIA) and the Spanish secret service."