WASHINGTON — Federal regulators proposed tougher standards on nontraditional mortgage loans yesterday, saying that expanded use of new interest-only and adjustable-rate loans has put both borrowers and the financial system at greater risk.

The proposed guidelines come amid growing concerns that many borrowers have signed up for mortgages they cannot afford, setting the stage for widespread defaults in coming years. Nationwide, mortgage debt has surged by about 70 percent since 1999, to nearly $8 trillion.

The widespread use of alternative loans — combined with relaxed standards in evaluating the creditworthiness of borrowers — "present unique risks that institutions must appropriately measure," the Federal Reserve said in a statement issued jointly with other regulators.

To ease the dangers, the federal agencies proposed requiring lenders to assess the ability of borrowers to repay the full burden of loans after low-cost introductory periods have expired.

Lenders also would be required to give borrowers "clear and balanced information" about "relative benefits and risks."

The affected loans include popular "payment option" adjustable rate mortgages, in which borrowers receive loans with low initial teaser rates that adjust higher over time, and so-called interest-only loans in which borrowers don't pay any principal for several years.

An analysis by Deutsche Bank found that $83 billion in loans with adjustable rates were subject to hikes this year.

Federal officials are also concerned that alternative loans often are marketed to people with weak credit histories, the Fed said in a joint statement with the Office of Comptroller of the Currency, Federal Deposit Insurance Corp., Office of Thrift Supervision and National Credit Union Administration.

They proposed that lenders: