Those who died in Ondo state suffered from blurred vision and headaches, and then lost consciousness before dying within 24 hours.
A spokesman for the WHO said tests done so far had been negative for viral and bacterial infections.
He said the current theory was that the deaths were caused by weedkiller.
The outbreak started in the town of Ode-Irele.
All of those affected started showing symptoms between 13 and 15 April.
The WHO spokesman, Gregory Haertl, tweeted: "Current hypothesis is cause of the event is herbicides" and "Tests done so far are negative for viral and bacterial infection."
The tests were carried out at the Lagos University Teaching Hospital, the WHO said.
The World Health Organisation (WHO) says pesticide poisoning is the probable cause of 18 mysterious deaths in south-western Nigeria.