At least 50 hippos and other large animals have been killed by anthrax poisoning in Africa's oldest national park, its director has said.

Photos shared by the Virunga National Park in the Democratic Republic of Congo show the motionless animals floating along the Ishasha River. Park officials say they first started appearing last week.

The exact cause of the poisoning is unknown but tests have confirmed the presence of anthrax.

Park director Emmanuel de Merode said work was under way to recover the animals and bury them to prevent further spread, but it was difficult because there were no excavators.

"It's difficult due to lack of access and logistics," he told Reuters news agency.

"We have the means to limit the spread by... burying them with caustic soda."

The river runs north to Lake Edward, one of Africa's great lakes, where more dead animals have been reported locally.

Anthrax is caused by Bacillus anthracis and can be deadly but usually it does not spread easily.

It largely survives as spores that hide away in soil for years before entering an animal through inhalation or a cut or wound.

The Congolese Institute for the Conservation of Nature has warned residents to avoid wildlife and boil water from local sources before drinking.

Stretching 7,800 sq km (3,000 sq miles), Virunga is one of the most biologically diverse, but also most dangerous, national parks on the continent.

The park is a popular tourist destination but has also been affected by the conflict between rebel groups and the Congolese army in recent years.

Thousands of armed soldiers belonging to different rebel groups roam Virunga and its surrounding areas, battling for control of the region's rich resources.

Many rangers have died protecting wildlife there.

In recent decades there have been efforts to increase the number of hippos in the park after the population declined from over 20,000 to just a few hundred as a result of poaching and war.