Evidence suggests e-cigarettes help smokers quit, but more research is needed to confirm this and find out if "vaping" nicotine is better than using patches or gum, scientists said on Wednesday.

In an international review of available evidence, researchers found a paucity of robust scientific studies on e-cigarettes and their capacity to help people stop smoking, but said data so far pointed at likely benefits.

"Although our confidence in the effects of electronic cigarettes as smoking cessation interventions is limited because of the small number of trials, the results are encouraging," said Peter Hajek, a professor of clinical psychology and a member of the research team at the Cochrane Review, a respected research network that determines relative effectiveness of different health interventions.

The uptake of e-cigarettes, which use battery-powered cartridges to produce a nicotine-laced vapor, has rocketed in the past two years, but there is fierce debate about them.

Because they are relatively new, there is a lack of long-term scientific evidence on their safety. Some experts fear they could be a gateway to tobacco smoking, while others say they have enormous potential to help millions of smokers around the world kick their deadly habit.

The Cochrane Review's study, a so-called meta-analysis, drew on two randomized trials covering 662 smokers, and also considered evidence from 11 observational studies, to examine the effects of e-cigarettes on quit rates and on helping people cut down their cigarette smoking by at least half.

It also looked at side effects reported by e-cigarette users and found no evidence of serious problems.

The results showed about 9 percent of smokers who used e-cigarettes were able to stop smoking at up to one year, compared with around 4 percent who used placebo e-cigarettes.

Data on reducing smoking in people who did not quit showed that 36 percent of e-cigarette users halved the number of conventional cigarettes they smoked, compared with 28 percent of placebo users.

Only one of the trials compared e-cigarettes' quit success rate with patches, and this suggested they were about equal.

Robert West, director of tobacco research at University College London, said in an emailed comment that the findings suggested e-cigarettes could be a valuable public health tool.

"It's early days but so far it seems that these devices are already helping tens of thousands of smokers to stop each year."