WWF staff have been telephoned with death threats for opposing oil exploitation in Africa's oldest national park, which is home to one in four of the world's estimated 800 remaining mountain gorillas. It follows the attempted assassination of the Virunga national park's chief warden last month and the death of two Congolese park wardens in the last few months.

"We are taking the calls seriously," said a spokeswoman at the conservation group's Swiss HQ. "Two anonymous callers made specific threats. They were angry about a staff member's public statements about the negative impacts of oil. One caller said: 'We want his head'."

Reports of intimidation are said to have increased in the weeks since the Virunga chief warden, the Belgian aristocrat Emmanuel de Merode, was shot in an ambush by three people while driving alone in a park vehicle in April. "The callers … said that they had missed killing de Merode, but would not miss WWF's employee," said the WWF spokeswoman.

The fishing village of Kavanyongi on the northern shores of Lake Edward. WWF has said that oil exploitation could put at risk the livelihoods of 50,000 families that depend on the lake for jobs, food and drinking water. Photograph: Brent Stirton/Getty Images

Tensions are rising in Virunga as UK-based oil company Soco International PLC starts six weeks of seismic tests in Lake Edward. If successful, and subject to Democratic Republic of Congo (DRC) government approval, this can be expected to lead to exploratory drilling, possibly starting next year.

Soco, which received permission from the DRC government to explore for oil in the park in 2010, has been strongly opposed by environmental and civil society organisations as well as the UK government. The company has stated it will not seek to operate in the mountain gorilla habitat, the Virunga volcanoes or equatorial rainforest. [see footnote]

However, WWF has said that oil exploitation could put at risk the livelihoods of 50,000 families that depend on the lake for jobs, food and drinking water. Its country director in Congo DRC, Raymond Lumbuenamo, has said that security in and around the park is likely to deteriorate further if Soco goes ahead with its exploration plans.

"The security situation in the park is already bad. The UN is involved with fighting units and the M23 rebel force is inside the park. Oil would be a curse. It always increases conflict. It would attract human sabotage. The park might become like the Niger delta. Developing Virunga for oil will not make anything better. When you take part of the land [for oil] you put more pressure on the rest," he told the Guardian last year.

The park, which stretches over two million acres of forests, swamps, savannahs, snowfields and includes several active volcanoes, is the oldest in Africa, founded in 1925 by Belgium's King Albert I. It has been a battleground for decades and has been occupied by a succession of Congolese and Rwandan militia groups, including the M23 rebel force, the Hutu paramilitary Interahamwe group, and the armies of several countries. An estimated 140 conservation wardens have been killed and many more have fled or left since 1996.

De Merode, who is recovering in Nairobi, said in a statement: "Unfortunately the attack is not an uncommon incident for Virunga national park. Our rangers are targeted frequently due to their difficult work in protecting the park and its many valuable resources. They continue to face such risks to restore peace and the rule of law to the area and the people in their care."

• It was not our intention to imply and we are accordingly happy to make clear that Soco had no involvement in the alleged telephone calls made to WWF, or the assassination attempt on Mr de Merode. This article was amended on 22 May 2014 to make clear exploratory drilling requires DRC government approval.