The largest study to date on links between mobile phone use and certain types of brain cancer has proved inconclusive, researchers say.
The results indicate a possible health risk from heavy mobile phone use, said the World Health Organisation (WHO), which conducted the study.
But it said further research was needed for more conclusive answers.
The 10-year study of 13,000 people has been criticised because mobile phone companies provided 25% of the funding.
"The study doesn't reveal an increased risk, but we can't conclude that there is no risk because there are enough findings that suggest a possible risk," the study's chief author, Elisabeth Cardis, told AFP news agency.
The study looked at both healthy users of mobile phones and those with two types of brain cancer - glioma and miningioma tumours.
The heaviest phone users were reported to have a higher risk of both types of cancer but the researchers said "biases and errors" in the study prevented making a causal link.
Some data also suggested that overall, mobile phone users had a lower risk of brain cancer than people who do not use one.
But the researchers said problems with the study's methodology meant the finding was unreliable.
The director of the WHO's International Agency for Research on Cancer, which co-ordinated the study, said changing patterns of mobile phone use and lower emissions from handsets since the research began in 2000 meant further investigation into the phones and brain cancer was needed.
The BBC's Imogen Foulkes in Geneva says that some medical experts are already claiming the study is flawed - because instead of monitoring participants, it asked them to try to remember exactly how much and on which ear they had used their mobiles phones over the past ten years.
And questions have been raised over industry influence on the study, because mobile phone companies provided almost a quarter of the funding to carry it out.
A bigger study of the health effects of mobile phone use, involving 250,000 participants over 20-30 years, was launched in the UK last month.