The Federal Housing Finance Agency said on Thursday that it would move to reduce the loan balances for thousands of borrowers who owe more than their homes are worth.

The agency, which regulates the government-controlled mortgage finance companies Fannie Mae and Freddie Mac, said about 33,000 people were expected to be eligible for the principal reduction program. It will be a one-time offering for seriously delinquent borrowers, to help them with what the agency's director, Melvin L. Watt, said "could well be their final opportunity to avoid foreclosure."

Homeowners must meet certain criteria to qualify, including having an outstanding principal balance of more than $250,000 and being more than 90 days delinquent on mortgage payments as of March 1.

The program will allow an opportunity for delinquent, underwater borrowers in these areas to avoid foreclosure and save their homes.

The finance agency has developed other policies to help such homeowners, including allowing them to buy back Freddie- or Fannie-backed foreclosed homes at their market values.

The agency on Thursday also unveiled changes intended to minimize foreclosures, including modifications in Fannie Mae and Freddie Mac rules for selling defaulted loans.

The companies must now instruct their debt buyers to evaluate borrowers for loan forgiveness or principal reduction options. Also, debt purchasers can no longer release and walk away from possessed, vacant properties.