A mysterious disease that kills patients within 24 hours has claimed at least 18 lives in a south-eastern Nigerian town, the government says.

"Seventeen people have died of the mysterious disease since it broke out early this week in Ode-Irele town," said Ondo state government spokesman Kayode Akinmade.

The disease, symptoms of which include headache, weight loss, blurred vision and loss of consciousness, killed the victims within 24 hours of their falling ill, he said.

Laboratory tests have so far ruled out Ebola or any other virus, Mr Akinmade added.

The World Health Organisation (WHO) said it had information on 14 cases with at least 12 dead.

"Common symptoms were sudden blurred vision, headache, loss of consciousness followed by death, occurring within 24 hours," spokesman Tarik Jasarevic said.

Another WHO spokesman, Gregory Hartl, told AFP that according to a preliminary report, all those affected began showing symptoms between April 13 and 15.

Mr Akinmade said health officials and experts from the government and aid agencies, as well as WHO epidemiologists, had arrived in Ode-Irele to search for answers.

"There is no patient of the disease in any hospital and the disease has not spread beyond the town," he said.

Ondo state health commissioner, Dayo Adeyanju, told AFP that he and his officials went on a "field visit" with the WHO, UNICEF and the Nigerian Centre for Disease Control (NCDC).

"This was basically a case search to unravel the cause [of the disease]," he said.

Mr Jasarevic said blood and urine samples had been taken from two victims and cerebrospinal fluid from another.

"All samples have been sent to Lagos University Teaching Hospital this morning, and results are still pending. Investigations are still ongoing," he said.

AFP