The World Wildlife Fund on Monday condemned "death threats" against two of its staff fighting oil exploration in a Democratic Republic of Congo nature park just weeks after the park's director was shot.

"Unidentified callers have threatened the personal safety of two employees working in the city of Goma," WWF said in a statement.

The staff have been involved in efforts to block oil exploration by UK company Soco International PLC in nearby Virunga National Park, an 800,000-hectare nature reserve that is among the oldest in Africa and home to rare and endangered mountain gorillas.

Referring to one of the campaigners, a threatening caller said, "We want his head," according to WWF.

The organisation said there had been an increase in intimidating calls, text messages and notes since the park's director Emmanuel de Merode was shot several times while driving in the region in April.

"The callers to WWF said that they had missed killing de Merode, but would not miss WWF's employee," the organisation said. "WWF insists that authorities in DRC do everything in their power to bring the perpetrators of these threats and de Merode's attack to justice."

There has been mounting worldwide opposition to oil exploration in Virunga, a UNESCO world heritage site that is thought to have the greatest biodiversity on the continent.

The UK government, Nobel laureate Desmond Tutu, and Virgin's Richard Branson have all spoken out against Soco's operations, which have recently involved seismic tests in Lake Edward

UNESCO says exploration would breach international conventions on protecting world heritage, to which DRC is a signatory.

But it fears laws could be changed by the Congolese government to allow oil concessions covering 85 percent of the park's territory to be exploited. Exploration permits have already been distributed to international firms, including Soco and France's Total.

Soco's deputy director Roger Cagle told AFP that the firm had been "informed of the news that two employees of the NGO ... were threatened as a result of their opposition to oil development.

"Soco condemns this behaviour and does not tolerate any act of threat or intimidation."

Local activists told AFP in October that they received regular threats bordering on harassment.