The "current hypothesis is cause of the event is herbicides", WHO spokesman Gregory Hartl said in a tweet.
"Tests done so far are negative for viral and bacterial infection," he added.
The victims began showing symptoms early last week in what Ondo state spokesman Kayode Akinmade called a "mysterious disease", prompting fears of a new infectious disease outbreak in a region ravaged by Ebola.
The victims, whose symptoms included headache, weight loss, blurred vision and loss of consciousness, died within a day of falling ill in the town of Ode-Irele, in southwestern Ondo state.
The Ondo state health commissioner, Dayo Adeyanju, told AFP on Saturday that 23 people had been affected.
Akinmade said health officials and experts from the government and aid agencies, as well as WHO epidemiologists, had arrived in Ode-Irele to investigate the deaths.
The tests were carried out at the Lagos University Teaching Hospital, the WHO said.
Source: Agence-France Presse
Comment: Despite the industry claims that such products are 'safe', the dangers posed by pesticide exposures are quite real. Some of their toxic effects include cancer, diabetes, Parkinson's, autism, infertility and birth defects, as well as nervous system damage.