Male rats exposed to very high levels of the kind of radiation emitted by mobile phones developed tumours in the tissues around their hearts, according to a draft report by US government researchers on the potential health risks of the devices.
Female rats and mice exposed in the same way did not develop tumours, according to the preliminary report from the US National Toxicology Program (NTP), a part of the National Institute of Environmental Health Sciences.
The findings add to years of research meant to help settle the debate over whether mobile phone radiation is harmful.
Although intriguing, the findings can not be extrapolated to humans, NTP scientists and the US Food and Drug Administration (FDA) said on Friday.
They noted that the animal studies were meant to test extreme exposures to mobile phone radiation, and that current safety limits on mobile phone radiation are protective.
However, the two 10-year $US25 million ($A35 million) studies - the most comprehensive assessments of health effects and exposure to radiofrequency radiation in rats and mice to date - do raise new questions about exposure to the ubiquitous devices.
In the studies, about six per cent of male rats whose entire bodies were exposed to the highest level of mobile phone radiation developed schwannomas - a rare type of tumour - in nerve tissue near their hearts, while there were no schwannomas in animals that were not exposed to radiation.
"The intriguing part of this is the kind of tumours we saw were similar to tumours noted for quite some time in some epidemiological studies in heavy duty mobile phone users," John Bucher, a senior scientist with NTP, said in a telephone interview.
"Of course, these were in the nerves in the ear and next to the brain, but the tumour types were the same as we saw in the heart."
Dr Otis Brawley, chief medical officer of the American Cancer Society, noted that the studies were negative for common tumours.
"These draft reports are bound to create a lot of concern, but in fact they won't change what I tell people: the evidence for an association between cellphones and cancer is weak, and so far, we have not seen a higher cancer risk in people," he said in a statement on Twitter.
Brawley said if mobile phone users are concerned about this data in animals they should wear an earpiece.
Unlike ionizing radiation such as that from gamma rays, radon and X-rays, which can break chemical bonds in the body and are known to cause cancer, radiofrequency devices such as mobile phones and microwaves emit radiofrequency energy, a form of non-ionizing radiation.