An international panel of experts has weighed in on the controversy about cellphone safety — and come to a conclusion that falls far short of recommending that consumers put down the devices, although it may make them more anxious.

Cellphones are “possibly carcinogenic” to humans, according to the panel organized by the World Health Organization. But an exhaustive, eight-day review of hundreds of studies concluded that the existing evidence is insufficient to know for sure. And because cellphones are so popular, further research is urgently needed, the experts said.

“Possibly carcinogenic” is the WHO’s third-highest rating, falling below “carcinogenic” and “probably carcinogenic” but above “not classifiable” and “probably not carcinogenic.” Other substances that the group has categorized as “possibly carcinogenic” include talcum powder, which has been possibly linked to ovarian cancer, and low-frequency magnetic fields, which are emitted by power lines and appliances and have been possibly associated with childhood leukemia.

Nonetheless, the cellphone classification marks a departure for the WHO, which previously said there were no risks from exposure to radio frequency electromagnetic fields emitted by the devices.

“The conclusion means that there could be some risk, and, therefore, we need to keep a close watch for a link between cellphones and cancer risk,” said Jonathan M. Samet of the University of Southern California, who chaired the WHO’s International Agency for Research on Cancer panel.

The classification was based primarily on two large epidemiologic studies that found an association between cellphone use and brain cancer, particularly a malignant form called glioma and a benign tumor known as acoustic neuroma.

The panel, which included 31 scientists from 14 countries, did not quantify the possible risk; nor did it estimate how much cellphone use might be safe or risky, make any recommendations about whether cellphones should be regulated more strictly, or recommend what steps consumers should take. But one panel member said users might consider common-sense precautions such as texting more instead of talking and using a headset to keep the phone farther from the head to minimize exposure.

“This is left to the individual consumer to make a decision about whether the current level of evidence is strong enough to take such precautionary measures,” Kurt Straif, who heads the IARC’s monographs program, said in a telephone briefing Tuesday.

But some experts said the conclusion should lead to immediate action, not only by consumers but also by the Food and Drug Administration and the Federal Communications Commission.

“This is the first formal acknowledgment that we may have a problem on our hands — and it could be a very big problem,” said Louis Slesin, editor of Microwave News, a trade publication. “The IARC decision, surely, is a wake-up call that people, especially children, should take sensible precautions.”

CTIA, which represents cellphone makers, dismissed the significance of the classification, noting that it was not based on new research.

“IARC conducts numerous reviews and in the past has given the same score to, for example, pickled vegetables and coffee,” CTIA spokesman John Walls said in a statement. “This IARC classification does not mean cellphones cause cancer. Under IARC rules, limited evidence from statistical studies can be found even though bias and other data flaws may be the basis for the results.”

The FDA and the FCC have previously concluded that there is no link, he noted. The FCC did not have any immediate comment. In a statement, the FDA said that it would review the new report but that the “existing weight of scientific evidence does not show an association between exposure to non-thermal radio frequency energy and adverse health outcomes.”

Cellphone makers have fought increased regulation, including attempts in Maine and California to require warnings to consumers about the radio frequency electromagnetic fields emitted by the devices. San Francisco sidelined such a plan this year after coming under intense pressure.

Some independent experts said the IARC had no choice but to categorize cellphones as a possible cancer risk, given the criteria it uses and the dearth of research in this area.

“IARC virtually never says that anything is not a carcinogen (they only have done so in one out of nearly 1,000 evaluations) and the evidence that these fields are carcinogenic is weak and unpersuasive,” Kenneth Foster, a professor of bioengineering at the University of Pennsylvania, said in an e-mail.

“Problem is that the studies were not really designed to identify long-term risks. Does anyone know what cellphone they used a decade in the past? This issue will be very difficult to resolve, unless some study in the future finds strong evidence of a problem, and there have been enough done already so that is unlikely in my view,” Foster said.

Panel members stressed that more research is needed to explore the question, especially given that an estimated 5 billion people use cellphones and that the number is expected to increase rapidly.

“This review represents an important first look by IARC at an exposure that is transforming the world,” Samet said.

But panel members acknowledged that such studies would be difficult because of how quickly the technology is changing. The two studies that primarily led to the group’s conclusion were based on people using what is now outdated technology, for example.

The experts added that it remains far from clear how cellphones could cause brain cancer, given that the electromagnetic radiation emitted by the devices appears to be far too weak to have a significant biological effect.

“We still have much to learn about how these fields interact with biological materials,” Samet said.

Other experts said that any risks from cellphones needed to be kept in perspective.

“People who talk while driving are already at greater risk of having an accident, and worrying about your cellphone will only increase that risk,” said Marc Siegel, an associate professor at the New York University School of Medicine. “We spend thousands of hours a month at our computers or on our cellphones and BlackBerrys, talking and texting. Even if the small amounts of radiation won’t kill us, the inactivity certainly might.”