The U.S. Food and Drug Administration is cracking down on teas, supplements, creams and other products that falsely claim to cure, treat or prevent cancer even though they are not agency-approved drugs. All are available for sale on the internet.

The agency has sent 25 warning letters to companies and individuals marketing these products, FDA officials said Tuesday.

FDA officials said the statements made about these products are dangerous because they could prevent a patient from seeking proper treatment for cancer. The products could also harm a cancer patient by interacting with other drugs the patient is taking.

"FDA is very concerned that consumers will purchase these products on the internet and use them instead of products that have been proven safe and effective," said Michael Levy, director of labelling in the agency's new-drug division.

The letters criticized unproven claims made about these products including the ability to "destroy the enzyme on DNA responsible for cancer cells," and the power to "neutralize" carcinogens.

In March, Canada's Competition Bureau launched Project False Hope, which targets cancer-related health fraud online. It announced it had uncovered dozens of Canadian-operated websites selling cancer-related products.

In the U.S., one product's website had a testimonial claiming it had cured a patient's skin cancer in three days, according to one of the letters.

The ingredients of these unproven treatments include bloodroot, shark cartilage, coral calcium, cesium, ellagic acid, and a variety of mushrooms.

Officials said that if the warnings are not heeded, the agency could take action including seizure of the products and criminal prosecution.

"Health fraud has been around for years, and it is a cruel form of greed," said David Elder, director of the agency's enforcement office. "Fraud involving cancer treatments can be especially heartless."