The UN health body also said the cigarette-shaped electrical devices should be banned from public indoor spaces, in series of recommendations ahead of a global meeting on tobacco control.
The devices, which have surged in popularity in particular among young people, function by heating flavoured nicotine liquid into a vapour that is inhaled -- much like traditional cigarettes, but without the smoke.
"The existing evidence shows that (e-cigarette) aerosol is not 'merely vapour' as is often claimed in the marketing of these products," the WHO said.
Despite a dearth of research on the health effects of "vaping" the UN agency said 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".
It said retailers should be prohibited from selling e-cigarettes to minors, and called for the scrapping of vending machines in almost all locations.
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-cigarette regulation could be agreed.
They also often come without the regulation that has increasingly dogged the traditional cigarette industry, with some countries allowing advertising and the sale of e-cigarettes to minors.
So far, users have also widely been permitted to freely vape in places where traditional smoking is strictly off limits.
Sales have risen sharply since e-cigarettes were first introduced in 2005.
Since then, the sector has ballooned from a single manufacturer in China to an estimated $3.0-billion global industry with one or more of 466 brands available in 62 countries, WHO said in a statement.
It voiced concern about the growing role of traditional tobacco companies in the once niche market.
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 said there was a lack of research backing up 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 stop smoking, it said.
It also warned of the "renormalisation effect" of e-cigarettes, meaning they can make smoking itself more attractive and "perpetuate the smoking epidemic".
"E-cigarettes have been marketed in almost 8,000 different flavours, and there is concern they will serve as a gateway to nicotine addiction and, ultimately, smoking, particularly for young people," the WHO warned.
It said e-cigarette use had at least doubled among both adults and teens from 2008 to 2012.
In 2012, seven percent of EU citizens over the age of 15 had given vaping a shot.
However, only one percent of the population in the bloc, which governs e-cigarette use in a fragmented fashion, indulged in the practice regularly, WHO said.
In the United States, where e-cigarettes are completely unregulated, they appear more popular, with 47 percent of smokers and former smokers saying last year they had tried the devices.
Only four percent in that group however said they vaped on a regular basis.
More concerning perhaps was a warning from US health authorities Monday that the number of US youths who have tried e-cigarettes tripled from 2011 to 2013.
More than a quarter of a million young people who had never smoked a cigarette used e-cigarettes in 2013, according to the US Centers for Disease Control and Prevention.
With sales of e-cigarettes expected to top $2.0 billion this year in the United States alone, the American Heart Association also called Monday for the devices to be regulated as regular cigarettes.
In light of their skyrocketing popularity, the WHO said Tuesday that e-cigarettes with fruit, candy or alcohol flavours should be banned "until it can be proven they are not attractive to children and adolescents".