The fierce debate over whether e-cigarettes can help people quit smoking took another twist on Monday as a research paper on their use by cancer patients was criticized as flawed.

The study of cancer patients who smoke found that those using e-cigarettes as well as tobacco cigarettes were more nicotine dependent and equally or less likely to have quit than those who didn't use e-cigarettes.

The scientists behind the research, which was published online in Cancer, the journal of the American Cancer Society, said their results raised doubts about whether e-cigarettes had any benefit in helping cancer patients to give up smoking.

But that conclusion was questioned by other tobacco and addiction researchers, who said the selection of patients for the study had given it an inherent bias.

The uptake of e-cigarettes, which use battery-powered cartridges to produce a nicotine-laced vapor for the "smoker" to inhale, has rocketed in the past two years, but there is fierce debate about their potential risks and benefits.

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

What few studies there are give a mixed picture, with some concluding that e-cigarettes can help people give up a deadly tobacco habit, while others suggest they may carry health risks of their own.

A World Health Organization (WHO) report last month called for stiff regulation of e-cigarettes as well as bans on indoor use, advertising and sales to minors.

But that report itself was also criticized by experts who said it contained errors, misinterpretations and misrepresentations.

For the Cancer journal study, researchers led by Jamie Ostroff of the Memorial Sloan Kettering Cancer Center in New York City studied 1,074 cancer patients who smoked and who were enrolled between 2012 and 2013 in a tobacco treatment program at a cancer center.

They found a three-fold increase in e-cigarette use from 2012 to 2013 - rising from 10.6 percent to 38.5 percent.

At enrolment onto the program, the researchers' analysis found, the e-cigarette users were more nicotine dependent than non-users, had more prior quit attempts, and were more likely to be diagnosed with lung or head and neck cancers.

By the end of the study period, the researchers said, e-cigarette users were just as likely as non-users of e-cigarettes to be smoking.

But Robert West, director of tobacco research at University College London, said the study was not able to assess whether or not for cancer patients who smoke using an e-cigarette to try and quit is beneficial "because the sample could consist of e-cigarette users who had already failed in a quit attempt, so all those who would have succeeded already would be ruled out".  

Peter Hajek, director of the Tobacco Dependence Research Unit at Queen Mary, University of London, agreed that the study's data did not justify the conclusions.

"The authors followed up smokers who tried e-cigarettes but did not stop smoking, and excluded smokers who tried e-cigarettes and stopped smoking," he said.

"Like smokers who fail with any method, these were highly dependent smokers who found quitting difficult. The authors concluded that e-cigarette (use) was not helpful, but that would be true for any treatment however effective if only treatment failures were evaluated."