A Congolese rebel group accused of killings, mass rapes and other atrocities is taking groups of tourists on gorilla treks to the Virunga national park and using the proceeds to fund its insurgency.

Government authorities were forced to withdraw from parts of the park earlier this year because of the advance by M23 rebels, and now tourists wanting to see the rare mountain gorillas are being led by rangers who have defected to the militia.

A tour operator based in Bunagana, a border town that has been held by the M23 since early July, said international tourists were still coming to the park, at a cost of more than $350 (£218) a day.

"We had 39 visitors in August alone," said Laurent Ntawukiruwe, director of Shoushou Safari Tours. "I have agents in many different countries that help to bring tourists in. We have had German and Italian visitors, and soon we will have some French coming." A group of British tourists were booked to visit on 29 October, but cancelled their reservation.

Ntawukiruwe said his tours were run by rangers who had defected to M23, which is fighting an insurgency against the Democratic Republic of the Congo government. "The four rangers … are now in collaboration with M23," he said.

In September, a report by Human Rights Watch accused the M23 fighters of committing "widespread war crimes, including summary executions, rapes, and forced recruitment". The UN has accused neighbouring Rwanda of supporting the rebels, prompting a number of countries to freeze aid to President Paul Kagame's government.

The outbreak of hostilities between M23 and the Congolese army in May forced the Congolese National Park Authorities (the ICCN) to shut down its tourism activity. Shoushou Safari Tours has an ICCN sign on display at its Bunagana office, but the operation has no link to the legitimate park authorities, said Virunga park director Emmanuel de Merode.

"These tours have absolutely nothing to do with us," said De Merode. "This puts the gorillas and the visitors at risk. You need to have well-trained staff to lead these tours – handling mountain gorillas is an extremely sensitive issue. These expeditions are detrimental to the park and we condemn them in the strongest terms."

Vianney Harakandi in the Virunga park tourism office in Goma confirmed that a handful of rangers had defected to join M23. He criticised Shoushou Safari Tours for trying to suggest the expeditions were official, saying it was an attempt to disguise the fact that the tours were aiding M23. "Ultimately the money the tourists pay will just reinforce the capacity of this illegal armed group," he said.

Eugene Rwabuhihi, from the M23 department of tourism, environment and conservation, suggested the rebels were planning to step up the operation. "We are preparing for tourism activities and securing the gorilla population," he said. "When tourists come, they will pay to see the gorillas and we will receive part of that money for providing their security."

Tourists pay $300 for the permit to see the gorillas, $50 for transportation and $10 for the guides. It is unclear how much of this goes to the rebel group. But it is significantly cheaper than visits to see mountain gorillas in Uganda and Rwanda, where permits alone cost $500 and $750 respectively.

Tourists wanting to see the gorillas usually fly to the Ugandan capital, Kampala, before driving to the Congolese border and crossing into Bunagana, where they stay in the Mountain Gorilla hotel. The hotel has recently hosted meetings of high-ranking M23 military commanders.

Because the border crossing is controlled by M23, the tourists no longer have to obtain a Congolese visa. "It is much simpler for me like this, because I don't have to pay for my visa, or organise all sorts of different permits and passes," said Abdel, a French tourist who has booked with Shoushou and did not want to reveal his surname. "I am fascinated by the Congo, but I've never been. I'm going into a war zone and there is a lot that I don't know. I'm going there to learn, to get that knowledge."

Abdel, who lives in Kampala, said he was not concerned that the tours were being conducted by, and make profits for, an armed rebel group. "I want to be able to see the gorillas without lots of other tourists around, but this is also an experience that will allow me to see many things I've never seen before.

"I am keeping an open mind. The situation there is complicated; it's not as simple as saying 'the rebels are bad, the government is good'.

"My father fought in the Algerian resistance against France, and back then they called him a terrorist. But he was a freedom fighter."

National park plagued by death

Virunga is the oldest and most bio-diverse national park in Africa, but it has been ravaged by the cyclical violence that has plagued eastern Democratic Republic of the Congo over the last 25 years.

The fall of Mobutu Sese Seko in the 1980s brought violence and poachers. Animal populations, especially of large mammals, were severely affected. The Rwandan genocide and subsequent Congo wars brought further instability, with the rebel Democratic Forces for the Liberation of Rwanda establishing itself in the centre of the park.

This instability culminated in the Kivu conflict of 2004 to 2009, where Laurent Nkunda's National Congress for the Defence of the People rebels built their strongholds in and around Virunga.

His fighters finally signed a peace agreement in March 2009 and integrated into the Congolese army, allowing the Congolese National Park Authorities to oversee the recovery of Virunga's plants and wildlife, especially the rare mountain gorillas. Tourists had begun to return in increasing numbers; in 2009 the park received 550 visitors, by 2010 that figure rose to 3,000, and was projected to reach 6,000 this year.

But in April those former fighters mutinied again, forming M23. The M23 commanders have taken key towns in and around the park, and for now it is largely under M23 control.

Park authorities have been forced to suspend tourism again, and rangers risk their lives as they seek to ensure the security of the local Congolese people and Virunga's mountain gorillas. More than 130 rangers have been killed since 1996, including one in May.