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.

The WHO acknowledged that e-cigarettes were "likely to be less toxic for the smoker than conventional cigarettes".

But it stressed that very little research had so far been done to prove the safety of the devices or to boost manufacturers' claims that they can help smokers quit traditional cigarettes.

And it 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".

The UN agency'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 will likely be agreed.

TimesLive