The government of Equatorial Guinea has freed four South African mercenaries jailed for plotting to overthrow President Teodoro Obiang Nguema, South Africa's foreign ministry has announced.

The news followed the revelation in London's Daily Telegraph earlier Tuesday that Obiang had pardoned the mercenaries' leader, Briton Simon Mann, on humanitarian grounds.

South Africa's Department of International Relations and Cooperation said in a statement that the four South Africans, whom it did not name, had been released into the custody of the South African Embassy in Malabo.

The statement said South Africa "acknowledges the humanitarian gesture by the government of Equatorial Guinea."

An hour earlier, the department had announced that President Jacob Zuma would  undertake a working visit to Equatorial Guinea on Wednesday.

It said in that announcement: "South Africa is keen to promote economic relations in the areas of agriculture, mining, energy, tourism and infrastructure development, in addition to collaboration in the furtherance of the African political agenda including the strengthening of democracy in the continent."

The Daily Telegraph reported that the decree pardoning Mann said he had been released given his need "to receive regular medical treatment and to be with his family". Mann was reported to have undergone a hernia operation while in prison, the newspaper said.

In 2008, Mann was sentenced to 34 years in jail by a court in Equatorial Guinea for his part in an attempted coup.

He had been extradited to the country by Zimbabwe after being jailed there in 2004. He and nearly 70 other mercenaries were arrested in Harare when their aircraft stopped over there on its way to Malabo to execute the coup.