Electronic cigarettes should face greater restrictions on their use, sale and promotion, the World Health Organisation has said.
Although less harmful than real cigarettes, the nicotine vapour inhalers carry a passive smoking risk that mean they should be banned indoors, the UN organisation said.
It also said they were dangerous to children and recommended a clampdown on sales to minors with vending machines removed "in almost all locations".
There is only "limited" evidence that e-cigs help smokers quit, which "does not allow conclusions to be reached", it added, calling for a ban on marketing them as "smoking cessation aids" without more evidence.
The British Medical Association welcomed the report, released ahead of October's WHO Framework Convention on Tobacco Control in Moscow. But health charity Action on Smoking and Health (Ash) said it opposed including e-cigarettes in "smoke-free" legislation.
In the report on electronic nicotine delivery systems (ENDS), the Geneva-based health body said: "The fact that ENDS exhaled aerosol contains on average lower levels of toxicants than the emissions from combusted tobacco does not mean that 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."
Regarding the claims they help people quit smoking, it added: "Although anecdotal reports indicate that an undetermined proportion of ENDS users have quit smoking using these products, their efficacy has not been systematically evaluated yet.
The one "randomised control trial" that has been carried out found that ENDS were about as effective as nicotine patches, it added.
Electronic cigarettes are currently regulated as consumer products in the UK but from 2016 any nicotine-containing products (NCPs) which make medicinal claims - such as claiming they are a stop-smoking aid - will be regulated by the Medicines and Healthcare Products Regulatory Agency.
Dr Ram Moorthy, deputy chairman of the BMA's Board of Science committee, said the report was encouraging and that tighter regulation was needed to prevent the "normalisation" of smoking.
He said: "There is a need for research to understand the health impacts of e-cigarettes on both the user and bystanders, and it is vital that the sale of e-cigarettes is appropriately regulated to ensure they are not sold to minors, and are not aggressively marketed to young people as tobacco was in the past.
"Any health claims must be substantiated by robust independent scientific evidence to ensure that the consumer is fully informed regarding potential benefits and risks of e-cigarettes."
But Ash said that regulation needed to be proportionate, with Hazel Cheeseman, its director of policy and research, saying there was "no evidence of any harm to bystanders from use of these devices".
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 that electronic cigarettes are completely safe, as the WHO acknowledges, they are considerably less harmful than smoking tobacco and research suggests that they are already helping smokers to quit.
"The planned regulatory approach in the UK is consistent with the suggestions made by the WHO and balances risks and opportunities of these products.
"It is hoped that our regulatory regime will promote the development of safer and more effective products while ensuring that 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 which will ban most advertising, limit nicotine levels and set standards for ingredients, labelling and packaging."