"It's 100 percent false to affirm this is a therapy for smokers to quit," WHO anti-tobacco official Douglas Bettcher told journalists.
"There are a number of chemical additives in the product which could be very toxic," he warned.
Electronic cigarettes are usually made of stainless steel and have a chamber for storing liquid nicotine in various concentrations. Users puff on them as they would a real cigarette but do not light them -- rather the cigarettes produce a fine, heated mist which is absorbed into the lungs.
The WHO is particularly aggrieved that some manufacturers have implied the organisation views it as a legitimate nicotine replacement therapy, like nicotine gum, lozenges and patches.
"Manufacturers of this electronic cigarette around the world have included WHO's name or logo, for example on their website, on package inserts or on advertisements," Bettcher said, without naming any company or manufacturer.
The electronic cigarette was first developed in China in 2004, and is now sold in several other countries including Brazil, Britain, Canada, Finland and Turkey.
Bettcher called on the manufacturers and marketers to conduct clinical studies and toxicity analyses and operate within the "proper regulatory framework".
"Until they do that, WHO cannot consider the electronic cigarette to be an appropriate nicotine replacement therapy, and it certainly cannot accept false suggestions that it has approved and endorsed the product."