Electronic cigarettes are harmful both to users and bystanders exposed to the fumes, the World Health Organization has said in a report warning they can damage growing foetuses and impact teenagers' brains.

The vaping devices have been pushed by producers as well as some governments as a safer alternative to smoking, and as a path to kicking the habit.

But in a strongly-worded series of questions and answers about e-cigarettes, the United Nations health agency said there was not enough evidence to say they help smokers quit, but there was clear evidence they are dangerous.

"There is no doubt that they are harmful to health and are not safe," the WHO said in one answer, stressing though that "it is too early to provide a clear answer on the long-term impact of using them or being exposed to them."

The battery-powered devices that enable users to inhale addictive nicotine liquids and exhale vapour "are particularly risky when used by adolescents," the organisation said, strengthening initial warnings it made last year.

"Nicotine is highly addictive and young people's brains develop up to their mid-twenties," it said, stressing that "exposure to nicotine can have long-lasting, damaging effects."

This is of high concern amid the increasing popularity of vaping among young people in many countries.

At the same time, there is little indication that using e-cigarettes is keeping teenagers away from more harmful products, with the WHO pointing out that those using the devices are more likely to smoke conventional cigarettes.

'Significant risks' for pregnant women

The WHO said that "for pregnant women, ENDS (electronic nicotine delivery systems) pose significant risks as they can damage the growing foetus."

Amid growing fears over serious health consequences linked to e-cigarette use, the organisation warned that so-called vaping increases the risk of heart disease and lung disorders.

The United States especially has seen a major health crisis tied to vaping. An acute lung illness epidemic that has killed more than 50 people and sickened more than 2,500 in the US.

The illness was later linked to a substance called vitamin E acetate, which is used as a thickening agent for vaping products containing the main psychoactive substance of cannabis - THC - that are often sold on the black market.

At least five other countries have initiated investigations to identify cases of lung injuries related to e-cigarette use, the WHO said.

The UN health agency also said that second-hand exposure to e-cigarette fumes was harmful, pointing out that the aerosols "typically contain toxic substances, including glycol which is used to make antifreeze."

"ENDS pose risks to users and non-users," it said, demanding tight regulation of the products, including bans on marketing to young people and on use in indoor workplaces and public spaces.

Critics have hit out at the findings, accusing the WHO of "anti-vaping activism".

A comment retweeted by tobacco company Philip Morris said the document was filled with errors and that the authors "should take responsibility for using blatant misinformation to prevent smokers from switching to a much less risky alternative."