Advertisers should steer clear of saying their products are “environmentally friendly” or “eco-friendly,” the Federal Trade Commission said in proposed rules put out Wednesday.

The agency, however, did not weigh in on other terms — “sustainable,” “natural” and “organic” — because they are the purview of the Agriculture Department.

The FTC, which has been concerned by unsubstantiated environmental claims in advertising, urged marketing companies to stay away from broad environmental claims such as “environmentally friendly” because few companies can live up to them.

“Very few products, if any, have all the attributes consumers seem to perceive from such claims, making these claims nearly impossible to substantiate,” the FTC said in a statement announcing changes to its Green Guides, which were first issued in 1992 and last revised in 1998.

Companies should also avoid making claims that the products are made with “renewable materials” or “renewable energy” when any oil, coal or other fossil fuel was used, the FTC said in a statement issued with the 229-page rules.

Dan Jaffe, chief lobbyist for the Association of National Advertisers, said he would be looking to see whether the FTC was correct in saying that consumers understand some green claims differently from companies.

“We will have to see if their conclusions are congruent with the ones that our members have discovered in the marketplace,” said Jaffe. ANA members spend an estimated $250 billion on advertising per year. “Advertisers want a fair marketplace. If people are stepping over the line, they (advertisers) want that stopped because it will disadvantage them in the marketplace.”

The FTC is seeking comments on the proposed changes. Comments will close Dec. 10, 2010, after which it will issue final rules.