One-quarter of the world's entire mountain gorilla population is under threat from sustained fighting in the Democratic Republic of Congo.

Just over 800 gorillas remain in the world, with 210 living in the Virunga National Park, a Unesco World Heritage site. A rebel army calling itself M23 entered the gorillas' habitat in the early hours of 8 May, and set up an operating base at Runyoni, a strategic peak in the Rutshuru territory, close to the border of Uganda. For more than two weeks, the rebels have been under siege as FARDC, the Congolese national army, fires rockets, mortars and anti-aircraft guns towards Virunga, Africa's oldest national park.

Lieutenant Colonel Vianney Kazarama, an M23 spokesman, confirmed yesterday that the FARDC has carried out aerial bombardments in the gorilla sector, including a helicopter attack close to Bukima, where a patrol post is situated. "Even today, the FARDC has just attacked with helicopters and six combat tanks," he said.

On 9 May, the rebels gunned down one Virunga park ranger and two Congolese soldiers. Twelve rangers have been killed on duty since the start of last year, and there are now four major rebel factions and a number of splinter groups operating in the park.

Emmanuel de Mérode, the park's director, says he does not know how the gorillas have fared. He and his team wake up most days to the sound of exploding ordinance. On Thursday, two patrol groups sent to Bukima were forced to pull back when they came under heavy fire.

The clashes, which have displaced at least 40,000 people since the beginning of April, are playing out in one of the most diverse habitats in the world – including Africa's two most active volcanoes and the world's largest lava lake, low-altitude equatorial forest, and high-altitude glaciers and snowfields.

When DR Congo was embroiled in a similar rebellion between 2007 and 2008, the CNDP, a precursor to the M23 rebels, took control of the park's headquarters and nine mountain gorillas were killed as a result of the insecurity. Since then, tourism at the park has rocketed from zero to 3,000 people a year, but once again it is down to zero as the park is closed to visitors.

Despite this, conservation efforts at Virunga have been successful and the gorillas' population has more than doubled since the late 1980s.

The current conflict began in April when, following calls for his arrest, one of the leaders of the FARDC's forces, General Bosco Ntaganda, defected. Ntaganda, nicknamed the Terminator, has been wanted by the International Criminal Court at The Hague for war crimes since 2006.

"He was one of the most powerful generals in eastern Congo, and now he's a man on the run," said Anneke Van Woudenberg, a senior researcher for Human Rights Watch in Goma. After Ntaganda defected, around 600 troops left their positions and followed suit. The mutiny was initially believed to be in response to calls for Ntaganda's arrest but M23 later denied this, citing instead poor living conditions and low wages as grounds for revolt.

While the fighting shows no sign of abating, the gorillas are now at the centre of a regional dispute, as DR Congo suspects Rwanda of arming the largely Tutsi rebels, the ethnic group that fought against the Rwandan genocide in 1994. Defending the rights of Rwandan Tutsis in DR Congo, where there is still widespread persecution by Hutus, and maintaining control of an illicit trade in metals and minerals, are among the reasons why Rwanda would chose to support M23.

Although M23 is isolated in Virunga's dense tropical environment, security experts agree that the rebel faction could not remain this strong without the aid of a regional power. "We still hold all of our positions which represent a large part of Rutshuru territory," said Lt Col Kazarama.

On a remote hilltop one hour's trek from the village of Kachiru overlooking Virunga, Major Jean-Claude Kifua of the FARDC confirmed that the rebels still hold five positions in addition to their Runyoni base, including Mbuzi mountain, overlooking the gorilla sector, and Tshanzu, just north of it.

A few kilometres north of the gorilla sector, overlooked by both the FARDC and rebels armed with anti-aircraft guns, Joseph Tibeshu and his wife Veronika Nylabitamu are two of three remaining residents in what was once a busy village. It is now a frontline. Too weak to flee, the couple are looking after a woman who is bedridden. The shell of a rocket-propelled grenade lies by the wall of his house. Joseph, who does not know his age, has seen many wars in his life, but says this is the worst. "They are killing people. The rebels are all around us here," he said.