U.S. researchers found "clear evidence" that cellphone radiation exposure can cause cancerous heart tumors in male rats.
It is still unclear what the final conclusions of their two-decades-long study of the health impact on rodents mean for humans.
Some evidence of links to brain and adrenal gland tumors was also found in male rats, but in female rodents and male mice signs of cancer weren't clear, the National Toxicology Program (NTP) concluded in its final report.
The program is run by the US Department of Health and Human Services and was tasked with reviewing the toxicity of mobile phone radiation in response to the devices' near ubiquity in modern life.
Radiation exposure in the trial was well above the levels most humans would experience, but researchers said the findings show the link between radio frequencies and tumours -- at least for rats -- "is real".
"The exposures used in the studies cannot be compared directly to the exposure that humans experience when using a cell phone," said Dr John Bucher, a senior scientist at the NTP.
"In our studies, rats and mice received radio frequency radiation across their whole bodies. By contrast, people are mostly exposed in specific local tissues close to where they hold the phone. In addition, the exposure levels and durations in our studies were greater than what people experience."
But he added: "We believe that the link between radio frequency radiation and tumours in male rats is real, and the external experts agreed."