The World Health Organization is investigating "another cluster of illness" in northern Congo, as a deadly mystery disease spreads in the region.

The World Health Organization speculated that a mass poisoning event was behind the deaths of 60 people. But African experts suspect a more mundane cause.

The outbreak began in January, with 419 cases recorded so far, and comes after another mystery illness killed 143 in December

Doctors are investigating more than 1,000 illnesses that have emerged since late January in five villages in Congo's Equateur province

More than 50 people have died in two clusters of unknown illnesses in northwest Congo, global health authorities said.