The WHO experts are on their way to Panzi, a locality in the southwestern Kwango province, where they will deliver essential medicines and diagnostic kits to help analyse the cause of the illness.
Laboratory tests are being conducted to determine the cause, the WHO said, adding that it would share more information as soon as it was available.
"Our priority is to provide effective support to the affected families and communities," said Matshidiso Moeti, the WHO's regional director for Africa.
"All efforts are underway to identify the cause of the illness, understand its modes of transmission and ensure appropriate response as swiftly as possible," she added.
The WHO said 394 cases and 30 deaths have been reported so far in the Panzi health zone, citing data from the Ministry of Public Health.
Earlier this week, local authorities said an unknown disease had killed 143 people in the region in November.
Symptoms of the illness include headache, cough, fever, breathing difficulties and anaemia, according to the WHO.
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