The statement, from Dr. Jeffrey Shuren, director of the F.D.A.’s center for devices and radiological health, also said, “Even with frequent daily use by the vast majority of adults, we have not seen an increase in events like brain tumors.”

The Federal Communications Commission sets exposure limits for radio-frequency energy from cellphones, but relies on the F.D.A. and other health agencies for scientific advice on determining the limits, the statement said.

For people who worry about the risk, health officials offer common-sense advice: Spend less time on cellphones, use a headset or speaker mode so that the phone is not pressed up against the head and avoid trying to make calls if the signal is weak.

Dr. Bucher noted that the radiation emitted increases when users are in spots where the signal is poor or sporadic and the cellphone has to work harder to connect.

In December, California issued advice to consumers about how to lower their exposure, including texting instead of talking, keeping the phone away from the head and body while streaming, downloading or sending large files; carrying the phone in a backpack, briefcase or purse, not a pocket, bra or belt holster; and not sleeping with the phone close to your head.

The two studies, involving 3,000 animals, are “the most comprehensive assessments of health effects and exposure to radio-frequency radiation in rats and mice to date,” according to a statement from the toxicology program, part of the National Institute of Environmental Health Sciences.

The studies extend partial findings released in May 2016, which found small increases in the incidence of tumors in the brains and hearts of male rats, but not female ones.