LAS VEGAS, Oct 20 (Reuters) - The regulator of Fannie Mae and Freddie Mac said on Monday it was developing rules to let Americans buy homes with down payments as low as 3 percent, part of a push to boost access to credit.

Mel Watt, director of the Federal Housing Finance Agency, said the new guidelines for the two taxpayer-owned firms would be "sensible and responsible."

"(They) will be able to responsibly serve a targeted segment of creditworthy borrowers with lower down payment mortgages," he said in a speech.

Watt said the lower down payments would be allowed when taking into account "compensating factors." He did not specify what those factors would be.

He also said the two firms, which guarantee most new mortgages in the United States, will change rules that will make it harder for them to force banks to repurchase faulty loans from the mortgage agencies.

(Reporting in Las Vegas by IFR analyst Albert Durso; Additional reporting by Jason Lange in Washington; Editing by Andrea Ricci)