Despite releasing vapour instead of smoke, the devices, officially known as Electronic Nicotine Delivery Systems (ENDS), still carry a risk to those standing around users, said a report for the Geneva-based UN organisation.
The report said: "The fact ENDS exhaled aerosol contains on average lower levels of toxicants than the emissions from combusted tobacco does not mean these levels are acceptable to involuntarily exposed bystanders.
"In fact, exhaled aerosol is likely to increase above background levels the risk of disease to bystanders, especially in the case of some ENDS that produce toxicant levels in the range of that produced by some cigarettes."
The report, to be discussed at a World Health Organisation (WHO) convention on tobacco control in Moscow in October, also recommends preventing manufacturers from marketing e-cigs as "smoking cessation aids" until they provide scientific evidence to back the claim.
The report also says they should be banned from sale to minors, and that vending machines should be removed "in almost all locations".
Electronic cigarettes are currently regulated as consumer products in the UK but from 2016 any nicotine-containing products (NCPs) that make medicinal claims - such as claiming they are a stop-smoking aid - will be regulated by the Medicines and Healthcare Products Regulatory Agency.
In its report, WHO said that so far the evidence e-cigarettes helped people quit smoking was "limited" and "does not allow conclusions to be reached".
It said: "Although anecdotal reports indicate an undetermined proportion of ENDS users have quit smoking using these products, their efficacy has not been systematically evaluated yet.
"Only a few studies have examined whether the use of Electronic Nicotine Delivery Systems is an effective method for quitting tobacco smoking."
The one "randomised control trial" that has been carried out found electronic cigarettes were about as effective as nicotine patches, it added.
Action on Smoking and Health (ASH), a campaigning public health charity, said it could not support any plans to include electronic cigarettes under smoke-free legislation.
Hazel Cheeseman, its director of policy and research, said there was "no evidence of any harm to bystanders from use of these devices" and said regulation needed to be proportionate.
She said: "Smoking kills 100,000 people in the UK alone. Smokers who switch to using electronic cigarettes in whole or in part are likely to substantially reduce their health risks.
"Although we cannot be sure electronic cigarettes are completely safe, as WHO acknowledges, they are considerably less harmful than smoking tobacco and research suggests they are already helping smokers to quit.
"The planned regulatory approach in the UK is consistent with the suggestions made by WHO and balances risks and opportunities of these products.
"It is hoped our regulatory regime will promote the development of safer and more effective products while ensuring electronic cigarettes continue to be widely available to those smokers who want to use them."
A Department of Health spokeswoman said: "More and more people are using e-cigarettes and we want to make sure they are properly regulated so we can be sure of their safety. We have already set out our intention to change the law to ban the sale of e-cigarettes to children under 18.
"The UK has an existing licensing system for higher strength products and those that claim to help people quit.
"We are also bringing in new European rules to cover lower strength products that will ban most advertising, limit nicotine levels and set standards for ingredients, labelling and packaging."