The World Health Organisation (WHO) warned against using electronic cigarettes, saying there was no evidence to prove they were safe or helped smokers break the habit.
First made in China and sold mainly over the Internet in countries including Brazil, Britain, Canada and Israel, they have grown in popularity despite a lack of regulatory approval, it said.
A typical electronic cigarette is made of metal tube with a chamber which holds liquid nicotine in a rechargeable cartridge. Users puff on it but do not light it, leading some to use it to evade smoking bans in public places, according to the WHO.
However, they inhale a fine mist of nicotine into their lungs, "plus potentially many other toxic compounds which we are not sure of", said to Douglas Bettcher, acting director of the WHO's Tobacco Free Initiative.
"The World Health Organisation knows of absolutely no scientific evidence whatsoever that would confirm that the electronic cigarette is a safe and effective smoking cessation device," Mr Bettcher told a news briefing.
"Toxicological tests and clinical trials have not been performed on this product," he said.
The electronic cigarette has yet to be shown to be a legitimate therapy like nicotine gum, patches or lozenges that help wean smokers from nicotine addiction, the UN agency said.
"If the manufacturers and marketers of the electronic cigarette want to help smokers to quit, then they should operate within proper regulatory frameworks, Mr Bettcher said. This meant rigorous clinical and toxicological studies must be carried out.
The WHO was greatly concerned that some manufacturers had used the WHO name or logo on their package or website, falsely implying endorsement, Mr Bettcher said, declining to name names.
The agency was contacting health authorities in its 193 member states to alert them of "these bogus, untested false claims". Turkey had already banned sales, he said.
The WHO agency had become aware only this year of the spreading use of electronic cigarettes worldwide.
"It has really taken countries and the WHO by surprise. It has been a product that appeared very suddenly on the market in a short period of time," Mr Bettcher said.
In 2003 WHO members clinched a treaty calling for stronger warnings on cigarette packages and limits on advertising and sponsorship. Some 160 countries have ratified the landmark pact.
Tobacco use is the single largest cause of preventable death worldwide, contributing to 5.4 million deaths from heart disease, stroke and other diseases annually, the WHO says.