A measles outbreak in the copper-mining 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 due to difficulties accessing remote areas, The U.N. Office for the Coordination of Humanitarian Affairs (OCHA) said in a draft report on the province's worst outbreak of the disease since 2010-11.

"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 the 2010-11 epidemic, 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 as they lie hundreds of kilometers south of the worst affected areas.

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

Mortality rates are low in developed countries but can rise to as high as 20 percent in poorer countries, according to medical charity Medecins Sans Frontieres (MSF).

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

The World Health Organization (WHO) warned last November that progress toward wiping out measles has stalled worldwide due to poor vaccine coverage.

Access to healthcare is low in Congo, which ranks 186 out 187 on the U.N. Human Development Index.