-->WASHINGTON (Reuters) - U.S. Treasury Secretary John Snow took direct aim Tuesday at mortgage finance firms Fannie Mae and Freddie Mac, repeating previous warnings to investors that government-sponsored enterprises are not financially backed by the U.S. government.

"We don't believe in a 'too big to fail' doctrine, but the reality is that the market treats the paper as if the government is backing it. We strongly resist that notion," he said in prepared remarks before a bankers group here.

"You know there is that perception. And it's not a healthy perception and we need to disabuse people of that perception. Investments in Fannie (FNM: Research, Estimates) and Freddie (FRE: Research, Estimates) are uninsured investments," he said.