Poaching has brought the hippopotamus population in Democratic Republic of Congo to within a few months of extinction, wildlife experts say.
The hippopotamus is now on the list of threatened species
Researchers from the Zoological Society of London (ZSL) say the population has halved in the last two weeks.
They say the Mai Mai militia has set up camp in Virunga National Park and catches the animals for meat and ivory.
The hippo entered the Red List of Threatened Species this year, and is declining in many parts of Africa.
ZSL says the militia killed hundreds of hippos in a two week period, and numbers now are below 400.
Its scientists warn the entire population may disappear before the end of the year without urgent action.
Twenty years ago there were about 22,000 hippos in Virunga Park, but the country's civil war brought numbers crashing down.
Its location on the border with Uganda and Rwanda has made it prone to incursions. Rwandan rebels fled to the area after the 1994 genocide.
The situation had stabilised after campaigns by Congolese and UN troops to oust the rebels and the establishment of an elite ranger corps within the park.
Some of these rangers have reportedly been attacked by the Mai Mai group.
ZSL's Bushmeat and Forests Conservation Programme Coordinator, Lyndsay Gale, who spends about half of her time in Virunga, said: "This is one of the biggest challenges the park rangers have had to face since the war.
"It comes as a devastating blow after recent surveys indicated wildlife populations were beginning to recover from over a decade of civil war."
As well as hippos, the militia is also hunting buffalo and elephant, ZSL reports. Ivory taken from elephant tusks and hippo teeth is sold for profit.
The society is appealing for more funds to go into its work with local conservation groups and rangers.
There are thought to be about 130,000 hippopotamus in Africa, with Zambia holding the biggest population.
But a continent-wide decline of about 30% in 30 years brought its inclusion on this year's Red List as a species vulnerable to extinction.