A measles outbreak in copper-rich Katanga province in the Democratic Republic of Congo has killed 315 people and infected at least 20,000, the United Nations said on Wednesday.

Hundreds more deaths have likely not been documented because of difficulties accessing remote areas, the U.N. Office for the Coordination of Humanitarian Affairs said in a draft report on the province's worst outbreak of the disease since 2011.

“The measles epidemic in the province of Katanga is only worsening and gaining ground,” said the report, seen by Reuters.

More than $2.4 million will be needed to organize vaccination drives and treat those already infected in the southeastern province, it said.

Some 1,085 people died and about 77,000 were infected in 2010 and 2011, according to a study in the scientific journal BMC Infectious Diseases.

The industrial copper- and cobalt-mining zones in the province — Africa's leading producer of both metals — have been largely untouched by the current outbreak because they are hundreds of miles south of the worst affected areas.

Measles is a highly contagious disease that can lead to deadly complications like diarrhea, dehydration, respiratory infection and encephalitis.

Mortality rates are low in developed countries but can rise as high as 20 percent in poorer countries, according to medical charity Doctors Without Borders.

It costs about $1 in developing countries to vaccinate a child against measles.

The World Health Organization warned last November that progress toward wiping out measles has stalled worldwide because of poor vaccine coverage.

Access to health care is poor in Congo, which ranks 186th out 187 nations on the U.N. human development index.

Reuters