Human Rights Watch called Wednesday on the Democratic Republic of Congo to investigate attacks and threats against opponents of a major oil exploration project at one of Africa's oldest national parks.

The group said activists and rangers at the Virunga National Park, home to some of the last surviving mountain gorillas rangers, had been detained by authorities and threatened or assaulted after criticising the project.

They include Emmanuel de Merode, the park's Belgian director, who was shot and seriously wounded by armed men on April 15 while driving alone.

De Merode had criticised the plan to explore for oil, saying it would have a negative impact on the park, its wildlife, and local communities.

"The attack on the national park's director was a painful and shocking reminder that people working to protect Africa's oldest park - its habitat, wildlife, and local communities - do so at enormous risk," said Ida Sawyer, senior Congo researcher at Human Rights Watch.

"Park rangers and activists should be able to oppose oil exploration in Virunga Park without risking their lives," Sawyer said.

British oil company SOCO International, which has a contract with the Congolese government to to explore for oil in and around the park, has condemned the use of violence and intimidation and denies any role. It said it would look into separate allegations of bribery, according to HRW.

HRW said a number of human rights and environmental activists who criticised the plans had received threatening text messages in the week after the attack on De Merode.

"You are playing with fire... it's useless to change your car because we know all the cars and we're everywhere you go with your team," said one.

"You think that by writing you're going to prevent us from extracting oil. You are going to die for nothing like de M�rode," said another.

Critics say the drilling would violate Congolese and international law. But many Congolese government officials support SOCO's plans for the financial benefits it would bring.

Created in 1925 during Belgian colonial rule, Virunga park covers nearly 800,000 hectares along the frontier with Uganda and Rwanda.

It is home to mountain gorillas and a small population of endangered lowland gorillas.