Federal regulators are casting a disapproving eye on mortgages that give borrowers low introductory rates but let them pile up more debt over the long run.

Starting this month, federally chartered lenders are being discouraged from qualifying buyers based on the low starter rates, when only the interest or a portion of the interest is due. Instead, they are being urged to evaluate the borrower's ability to pay for the loan at the full rate.

Regulators are trying "to add some discipline to the lending process," said Richard Wohl, president of Pasadena, Calif.-based IndyMac Bank. "Whenever you do that, you're going to have some (borrowers) that won't have the product available to them."

The tougher standard was issued in the form of "guidance" from the Office of the Comptroller of the Currency, the Office of Thrift Supervision, the Federal Reserve and other regulators. Guidance has less force than a regulation and provides no specific penalties for violation.

However, the regulators say they will "carefully scrutinize" lenders to see if they are following the new rules. Those who fail to do so, the guidance summary warns, "will be asked to take remedial action."

In addition, the guidance only applies to federally chartered lenders, including IndyMac, Countrywide Financial Corp., Washington Mutual Inc. and other behemoths. State-chartered banks, which are smaller but more numerous than federal banks, are not affected.

Industry observers are divided on the impact of the new guidance, which takes particular aim at loans often marketed as "option ARMs," in which the borrower has the option of choosing how much to pay on an adjustable rate mortgage.

In a letter to regulators last winter, IndyMac said 24 percent of the option loans it made in 2005 would be affected by the proposed tightening.

Last week, however, Wohl said that some of those borrowers would be eligible for other types of loans and that IndyMac's option ARM business was shrinking anyway as interest rates declined and customers moved to the certainty of fixed-rate loans.

Even so, interest-only and option ARM loans accounted for more than half of first-time mortgages and refinancings in California in July, according to the data-tracking firm First American LoanPerformance.

With an interest-only loan, borrowers pay only interest for a set period — typically three, five or 10 years. After that, the interest rate on the loan readjusts, and the borrower has to start paying the accumulated principal as well. The longer the interest-only period, the steeper the principal payments.

Option ARMs give borrowers the ability to put off both the principal as well as much of the interest for a period ranging from one month to several years. If a borrower pays the minimum in a weak housing market — the kind that began this year — his or her debt could grow faster than the value of the house.

That's the sort of thing regulators say they are acting to prevent.

Kathy Dick, deputy U.S. comptroller for credit and market risk, said interest-only and option ARMs originally were for a minority of savvy, well-off people whose income was variable — the self-employed and those who worked on commission or were paid intermittently.

"Now they're used to get someone into a home without a real analysis of their ability to pay," Dick said. "Lenders are qualifying people for homes they can't afford. We felt that wasn't consistent with prudent lending principles."