The World Health Organization called Tuesday on governments to ban the sale of e-cigarettes to minors, warning they pose a "serious threat" to unborn babies and young people.
The UN health body also said they should be banned from indoor public spaces, in a series of recommendations ahead of a global meeting on control of tobacco use later this year.
"E-cigarettes and similar devices pose threats to public health," said Douglas Bettcher, head of WHO's noncommunicable diseases unit.
"Regulation for these products must be given urgent attention," he told reporters in Geneva.
The number of users worldwide is thought to have doubled in the four years to 2012, the WHO said.
The devices -- which are particularly popular among young people -- function by heating flavoured nicotine liquid into a vapour that is inhaled, much like traditional cigarettes, but without the smoke.
They also often come without the regulations imposed on the traditional tobacco industry.
- Half WHO members don't regulate -
Half of WHO member states do not regulate the devices at all, in many cases allowing advertising and the sale of e-cigarettes to minors.
Users in some countries can also "vape" in places where smoking is banned.
But while manufacturers tout them as harmless aids to quit smoking, the WHO said there was little research to back those claims.
"The existing evidence shows that e-cigarette aerosol is not merely water vapour," said Bettcher.
And despite a dearth of research on the health effects of "vaping", the WHO 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 e-cigarette guidelines could be agreed.
- Soaring sales -
Since 2005, the e-cigarette sector has ballooned from a single manufacturer in China to an estimated $3.0-billion global industry, WHO said.
Bettcher voiced particular concern at the growing role of traditional tobacco companies.
"The tobacco industry pretends to be part of the solution to the global health disaster that they have created," he said.
While e-cigarette manufacturers claim they are helping smokers kick their habit, the WHO report said much of the marketing appeared to be targeting non-smokers.
The booming popularity of e-cigarettes could also lead to smoking regaining its lost respectability and help "perpetuate the smoking epidemic", it warned.
- Gateway to nicotine addiction -
It said the devices could "serve as a gateway to nicotine addiction" for young people.
The proliferation of e-cigarette flavours -- there are now nearly 8,000, including "strawberry milkshake" and "gummy bear" -- also raises the suspicion that youths are a prominent target, Bettcher said.
WHO said all e-cigarettes with fruit, candy or alcohol flavours should be banned "until it can be proven they are not attractive to children and adolescents".
The UN agency acknowledged that e-cigarettes were "likely to be less toxic" than conventional cigarettes, but said more research was needed.
Until manufacturers provide "convincing supporting scientific evidence," they should be banned from making health claims, including that e-cigarettes can help quit smoking, it said.
Armando Peruga, who heads the WHO's Tobacco Free Initiative, said most e-cigarette brands contain nicotine, while some also contain cancer-causing formaldehyde and even heavy metals.
While levels may be lower than in traditional cigarettes, it was unclear whether the health outcome would vary significantly, he warned.
US health authorities revealed Monday that the number of young people there 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 last year, the US Centers for Disease Control and Prevention said.
In the United States, where e-cigarettes are a completely unregulated $2 billion industry, 47 percent of smokers and former smokers said last year they had tried them, with four percent regular users.
In 2012, seven percent of EU citizens over the age of 15 had tried them, although only one percent of the population in the bloc indulged regularly, WHO said.