The UN health body also recommended that the cigarette-shaped electrical devices be banned from public indoor spaces "until exhaled vapour is proven to be not harmful to bystanders".
   
"The existing evidence shows that (e-cigarette) aerosol is not 'merely vapour' as is often claimed in the marketing of these products," WHO said.
   
The devices, which have grown in popularity in particular with youths, function by heating flavoured nicotine liquid into a vapour that is inhaled — much like traditional cigarettes, but without the smoke.
   
They also often widely come without the regulation that has increasingly dogged the traditional cigarette industry, with some countries allowing advertising and the sale of the devices to minors.
   
So far, users have also widely been permitted to freely "vape" in places where traditional smokings is strictly off limits.
   
WHO's recommendations came in a report published ahead of a meeting in Moscow in October of parties to an international convention on tobacco control, where new global guidelines on e-cigarettes will likely be approved.

Sales rising sharply

The sales of e-cigarettes have risen sharply since they were first introduced in 2005.
   
US health authorities warned Monday that the number of US youths who have tried e-cigarettes tripled from 2011 to 2013, with more than a quarter of a million young people who had never smoked giving the devices a shot last year.
   
Manufacturers maintain e-cigarettes are safe and claim they can help smokers quit traditional cigarettes.
   
But while the WHO acknowledged that the devices were "likely to be less toxic for the smoker than conventional cigarettes", it underlined the lack of research backing up the manufacturers' claims.
   
Until manufacturers provide "convincing supporting scientific evidence and obtain regulatory approval," they should be banned from making any health claims, including claims that e-cigarettes can be used to help buck smoking, it said.
   
It also warned of the "renormalisation effect" e-cigarettes can have, meaning they can making smoking itself more attractive and "perpetuate the smoking epidemic".
   
In the meantime, the UN body stressed that there was enough evidence "to caution children and adolescents, pregnant women, and women of reproductive age" about e-cigarette use, due to the "potential for foetal and adolescent nicotine exposure (having) long-term consequences for brain development".
   
Taking that into consideration, WHO said advertising should generally not be permitted of e-cigarettes, since that could make them attractive to children.