WASHINGTON (AP) _ The nation's major source of mortgage funds is introducing programs designed to overcome the largest obstacles to home ownership - lack of down payments and insufficient incomes.

The Federal National Mortgage Association announced Wednesday it will offer qualified low- and moderate-income families 25- and 30-year, fixed-rate mortgages that require down payments of just 3 percent.

Traditional home loans typically require down payments of between 10 percent and 20 percent.

The less expensive program ''addresses the single biggest problem first- time home buyers consistently identify - having enough money for the down payment,'' said James A. Johnson, chairman of the association also known as Fannie Mae.

Johnson said the Fannie 97 mortgages will be targeted at home buyers with household incomes of up to 100 percent of their area's median, except for designated higher cost areas. The median means half of the incomes are more and half are less.

Fannie Mae said the down payment on a $94,000 home using a Fannie 97 mortgage plan would be $2,820, compared with $4,700 for standard financing with a 5 percent down payment.

However, the monthly payment would be $734, compared with $719 for the standard mortgage. The calculations assume a 9 percent interest rate.

Fannie Mae hopes to provide $5 billion in Fannie 97 loans to an estimated 50,000 families in the first year.

The second program announced Wednesday will offer potential home buyers with modest incomes a 30-year, fixed-rate Start-up Mortgage with first-year payments lower than required for a standard fixed-rate loan.

Fannie Mae said the monthly principal and interest payments for a $94,000 home with 5 percent down would be $692 in the first year, gradually rising to $751 in the sixth year. Payments on a standard mortgage would be $719 a month for the life of the loan. The payments assume a 9.3 percent interest rate.

''We also know there are potential home buyers who are good credit risks, who have some funds accumulated for a down payment, but whose incomes keep them just out of reach of home ownership,'' Johnson said.

''For these consumers, the Start-up Mortgage, with its interest only payment in the first year and 5 percent down payment, allows them to qualify with lower payments and still have the benefit of a fixed-rate mortgage,'' he said.

Fannie Mae said it will require an estimated $9 in additional monthly mortgage insurance on loans with down payments of less than 15 percent. It said the amount will allow the company to offer more loans without affecting a consumer's ability to qualify for a mortgage.

Fannie Mae is a congressionally chartered, shareholder-owned company and the nation's largest source of home mortgage funds.

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EDITOR'S NOTE - Consumers seeking more information can call Fannie Mae's public information office toll-free at 1-800-732-6643.