WASHINGTON (AFP) - Coffee has been shown to reduce the risk of skin cancer by helping kill off damaged cells that could otherwise turn into tumors, according to a US study published on Monday.

The findings indicate that moderate caffeine drinking, or perhaps even applying coffee to the skin, could be useful in warding off non-melanoma cancer, the most commonly diagnosed of all skin cancers.

Using mice that had been genetically altered to suppress a protein enzyme called ATR, researchers showed that the mice were able to fend off cancer even when exposed to ultraviolet light.

Previous studies have suggested that drinking about a cup of caffeinated coffee per day has the effect of suppressing ATR and triggering the die-off of cells harmed by UV rays.

The altered mice eventually did develop cancer, but three weeks later than normal mice, according to the study published in the Proceedings of the National Academy of Sciences.

After 19 weeks of ultraviolet light exposure, the engineered mice showed 69 percent fewer tumors and four times fewer invasive tumors than the control group.

However, the protective effects only went so far. After 34 weeks of UV exposure, all the mice developed tumors.

"Eventually, if you treat them long enough, the mice will develop cancer so it is not 100 percent protection forever," Allan Conney, one of the study's authors, told AFP.

"Really, with almost any carcinogen, eventually all the animals will develop tumors," added Conney, who is director of the Susan Lehman Cullman Laboratory for Cancer Research at Rutgers University in New Jersey.

Conney and his team were able to confirm their hypothesis that caffeine -- when consumed or applied to the skin -- works by inhibiting ATR. Now they say more studies are needed to see how it may work on humans.

"We want to see whether caffeine has an effect in people when you give it topically," he said.

"Caffeine might become a weapon in prevention because it inhibits ATR and also acts as a sunscreen and directly absorbs damaging UV light."

Skin cancer is the most prevalent cancer in the United States, with more than one million new cases each year, according to the National Cancer Institute.

Non-melanoma types of skin cancer, including basal cell and squamous cell types, are the most commonly diagnosed and are often treatable if detected early.

Previous studies have shown coffee drinkers tend to have fewer incidences of breast, uterine, prostate and colon cancers, but the beneficial effects are not seen in people who drink decaffeinated coffee.