WASHINGTON — New home sales tumbled in August to the slowest pace in 17 years, while mortgage rates spiked this week, increasing pressure on the new chief executives of Fannie Mae and Freddie Mac to help stabilize the housing market.

Sales of new homes fell by 11.5 percent from July to August to an annual sales rate of 460,000 units, the slowest sales pace since January 1991, the Commerce Department said yesterday.

The median price slid 5.5 percent to $221,900.

The report came a day after the National Association of Realtors said existing home sales fell 2.2 percent last month to an annual rate of 4.9 million units, while the median sales price fell a record 9.5 percent to $203,100.

Many experts say the housing market's decline isn't over yet, as foreclosures and defaults continue to soar. Moody's Economy.com forecasts that U.S. home prices won't hit bottom for another year.

Also keeping buyers on the sidelines are tight lending standards and rising mortgage rates.

The average interest rate for a 30-year, fixed-rate loan this week is 6.09 percent, up from 5.78 percent last week, Freddie Mac said yesterday. That increase boosts the payment on a $200,000 loan, for example, by about $40 a month.

Freddie Mac and its sibling company, Fannie Mae, were seized by government regulators nearly three weeks ago as their losses mounted from the housing market's decline. Many in the mortgage industry have been anxiously waiting for the companies to scale back higher fees and tighter lending standards that the company put in place over the past year.

Since the takeover, "we have not seen any (mortgage) program changes, no assistance for clients to make anything different," said Jodi York-Caraballo, owner of Green Valley Mortgage Inc. in Bloomingdale, Ill. "They haven't eased up on lending restrictions."

Freddie Mac's new CEO, David Moffett said the government intervention is "providing needed confidence" to investors.