Two employees of WWF have received death threats in Democratic Republic of Congo because of the conservation group's opposition to plans by British company Soco International to search for oil in a national park, WWF said.
Soco's plans have drawn criticism from the British government and from environmentalists who fear they could damage Virunga National Park, the oldest and most bio-diverse in Africa.
Emmanuel De Merode, the park's Belgian director who was also publicly critical of Soco's plans, was shot and seriously wounded last month by unknown gunmen.
Soco has denied any link to that attack and said it condemned the latest threats. The firm says it can operate in the World Heritage Site using environmentally sensitive techniques.
Switzerland-based WWF, which has been one of the most vocal critics of the oil exploration, said that members of staff who had spoken publicly against the oil project had received anonymous threatening phone calls.
"Angered by a staff member's public statements about the negative impacts of oil, one caller said, 'We want his head'," WWF said in a statement posted on its website.
It said reports of intimidation against local civil society and anti-oil activists had increased since the shooting of De Merode, who is now recovering.
"Civil society activists tell WWF that they too have received menacing calls, text messages and notes. The callers to WWF said that they had missed killing de Merode, but would not miss WWF's employee," WWF said.
Investigations are continuing into the shooting.
Soco said at the time that any suggestion linking Soco to the attack was "completely unfounded, defamatory and highly inappropriate". The company also denied any link to the threats against WWF staff.
"Soco condemns this behaviour and does not condone acts of threats or intimidation of any kind," Roger Cagle, deputy chief executive, said in a statement sent to Reuters.
"It is emphasised that Soco is a responsible company operating under a strict Code of Business Conduct and Ethics, which it takes extremely seriously," he added.
The Congolese government supports Soco's right to explore for oil, arguing that it needs to know the value of the oil underneath Virunga before it can decide whether to allow the company to begin exploitation.