LOS ANGELES — Cellphone users may be at increased risk for two types of rare cancers and should try to reduce their exposure to the energy emitted by the phones, according to a panel of 31 international scientists convened by an agency within the World Health Organization.

Studies so far do not show definitively that cellphone use increases cancer risk, said the authors of the consensus statement issued Tuesday by the WHO. However, "limited" scientific evidence exists, they said, to suggest that the radiofrequency energy released by cellphones may increase the risk of two types of cancers: glioma, a type of brain cancer, and acoustic neuroma, a tumor of the nerve that runs from the ear to the brain.

Both types of cancers are rare: In the U.S., about 10,000 to 12,000 people develop a glioma each year and about 3,000 develop acoustic tumors. The elevated risk roughly doubles that risk after a decade of cellphone use, according to some studies. But the number of cellphone users worldwide — about 5 billion — means a potential cancer link should be taken very seriously, said Dr. Jonathan Samet, chairman of the department of preventive medicine at the University of Southern California’s Keck School of Medicine and the chairman of the panel that issued the report.

"What we have here is a warning from a public health point of view," Samet said. "We have half the world’s population already using cellphones, and people are using them younger and longer. We clearly need to keep track of this."

Other scientists said they remained skeptical of the link, which is mired in contradictory science, and that they found the decision by the WHO perplexing.

"I find the conclusions surprising given that there is increasingly strong evidence that cellphone use has no association with brain cancer occurrence," said David A. Savitz, a professor in the departments of epidemiology and obstetrics and gynecology at Brown University and a researcher on environmental exposures and health. "With few exceptions, the studies directly addressing the issue indicate the lack of association."

Cellphones were placed in a "possibly carcinogenic to humans" category by the International Agency for Research on Cancer, which develops scientific cancer-prevention strategies for the WHO. The agency’s other four categories for substances or agents are: carcinogenic to humans; probably carcinogenic to humans; not classifiable; and probably not carcinogenic to humans.

Scientists have long debated the potential cancer risk linked to cellphone use, but this statement marks the first time an independent group of scientists has taken anything other than a neutral stand.

"This is a major scientific consensus conference that has basically implicated cellphone radiation with increased tumor risk, " said Joel M. Moskowitz, director of the Center for Family and Community Health at the University of California, Berkeley’s School of Public Health and a long-time advocate of more research on the potential cellphone-cancer link. "I think they are particularly concerned about cellphones just because of the widespread utilization. It’s not like it’s some esoteric chemical used by industry that they think may be carcinogenic. Everyone is exposed to cellphones."

The panel based its conclusions primarily on data from the multi-country Interphone studies that were coordinated by IARC as well as research by Swedish cancer researcher Lennart Hardell. The Interphone data showed that people who used a cellphone 10 or more years had a doubled risk of glioma, a brain cancer that arises in the tissue surrounding and insulating brain cells. One study showed a 40 percent increase risk of gliomas for people who used cellphone an average of 30 minutes a day over a 10-year period. About 10,000 cases of glioma are diagnosed each year in the United States.

A 2004 study put the increased risk of acoustic neuromas at twice the normal risk after 10 years of cellphone use and higher for tumors on the side of the head where the phone is typically placed.

There is too little evidence to draw conclusions about other types of cancer, the report stated, including a 2009 study by Israeli researchers that linked cellphone use and cancer of the salivary gland.

But Savitz said the data are not compelling even for gliomas and acoustic neuromas. The more studies that are published on cellphones and health, he said, the more evidence accumulates that there is no increased cancer risk.

Many scientific questions remain, such as the lifetime risk of people who begin using wireless phones as children and just how cancer cells might arise from radiofrequency energy. But although the report will likely spur more calls for research, it’s not clear how much it will affect government policies, the cellphone industry or consumers, experts said.

Groups representing the wireless industry downplayed the significance of the report, noting that the WHO placed radiofrequency electromagnetic fields in the "possibly carcinogenic" category, along with about 266 other agents, including gasoline and occupational exposure to dry cleaning.

Coffee and pickled vegetables are listed as "possibly carcinogenic" also, noted John Walls, vice president for CTIA-The Wireless Association, in a statement issue Tuesday.

"This is not groundbreaking. It is a review of what already existed," Walls said in an interview. "It’s not the revelation that some would like to make it out to be."

The report acknowledged public interest in the issue and listed measures for consumers, such as using headsets, speaker phone or text messaging to reduce the amount of radiation reaching the brain. Radiofrequency energy drops off quickly, Moskowitz said: Moving a cellphone from one inch to 10 inches away from the head reduces radiofrequency energy 100-fold.

A summary of the panel’s findings will be released online at the WHO website and published in the July 1 issue of journal Lancet Oncology.

(Roan, a Los Angeles Times staff writer, reported from Los Angeles. Gabler, a staff writer at the Chicago Tribune, reported from Chicago.)