The U.S. Food and Drug Administration is cracking down on what are billed as fat-melting injections used in spas across the U.S., saying the drugs have not been proven safe or effective.

They are not approved by Health Canada.

Lipodissolve injections, a popular non-surgical alternative to liposuction, are used to dissolve small fat deposits around the legs, arms and belly. The FDA said Wednesday the drugs have not been cleared by federal scientists, as required by law.

The agency issued warning letters to six spas that offer the injections, citing them for making unsubstantiated claims about lipodissolve therapy.

"The claims made for your lipodissolve products are false and misleading in that they are not supported by substantial evidence or substantial clinical experience," states a letter to All About You Medspa in Madison, Ind.

Other spas cited by the FDA included: Pure Med Spa of Boca Raton, Fla., Monarch Med Spa in King of Prussia, Pa., and three others.

The website for Monarch Med Spa claims that, "Rather than go through the pain and discomfort associated with liposuction, patients now have the option of a series of injections with very minimal discomfort."

Calls to Monarch Med Spa were not immediately returned Wednesday.

FDA regulators called on the spas to stop using such claims and notify the agency within 15 working days and outline the steps they are taking to correct the violations.

"FDA is not aware of any credible scientific evidence to support these claims," said FDA deputy director Kathleen Anderson on a call with reporters.

Spas that offer the injections say they are safe and effective. But public safety advocates have called for proof and urge patients to think twice before paying thousands of dollars for an unproven procedure.

FDA said it has received reports of permanent scarring, hard lumps and dark spots on the skin that appear after the therapy.

Lipodissolve and similar treatments use two chemicals: phosphatidylcholine, or PC, and sodium dioxycholate, or DC. Those chemicals occur naturally in the human body, but that doesn't necessarily make them safe, said Lenox Hill Hospital plastic surgeon Dr. Jennifer Walden.

"They are used in the metabolic process of our bodies to break down fat, but they were never intended to be extracted, mixed with other ingredients and re-injected to break down fat," said Walden.

The FDA urged physicians who are using the drugs cosmetically to submit an approval application for regulatory review.