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The chief public health officer of the US, the Surgeon General, has called the use of e-cigarettes by children "a major public health concern".

In a report due to be released on Thursday, Vivek Murthy recommends more regulation and taxation.

His report agrees that e-cigarettes are less harmful than actual ones, but his concern is that the devices expose children to the risks of nicotine.

E-cigarettes are devices that turn nicotine liquid into a vapour.

Because they do not actually burn any material but leave users merely inhaling nicotine in steam, they are seen by some health experts as preferable to smoking cigarettes and, by some, as a way to give up cigarette smoking.

Dr Murthy's report says there is not enough evidence that prove e-cigarettes work in this way.

For young people, he says, e-cigarette use is strongly linked with the use of other tobacco products.

Dr Murthy says nicotine usage by young people risks mood disorders, attention deficits and addiction to nicotine that could lead to the use of traditional cigarettes.

Many tobacco companies are pinning their future on e-cigarettes and other alternatives as people continue to move away from using traditional nicotine products.

Last month, Marlboro maker Philip Morris launched a new product in the UK, Iqos, which it said could mean halting sales of its conventional tobacco products.