A mystery disease in the Democratic Republic of Congo is spreading mainly among children and putting severely malnourished people at risk, according to the World Health Organization (WHO), which sent experts to the region to investigate the outbreak.

In an update published on Sunday evening, the WHO said 406 cases of the undiagnosed disease were recorded between October 24 and December 5, 31 of which were deaths.

The symptoms of the disease, the cause of which has not been established, are fever, headache, cough, runny nose and body aches. It is spreading in the Panzi health zone in Congo's southwestern Kwango Province.

All severe cases were reported to be severely malnourished, and the majority of reported cases were children, particularly those aged under five, the WHO said.

"The area is rural and remote, with access further hindered by the ongoing rainy season," the UN's health agency added. "These challenges, coupled with limited diagnostics in the region, have delayed the identification of the underlying cause."

The statement said malaria is common in the area and could contribute to the cases. Experts say it is possible more than one disease is contributing to the cases.