WASHINGTON (AP) _ The National Aeronautics and Space Administration, under orders to cut its spending $5 billion by the end of the decade, announced a hiring freeze Monday and a stop in promotions at the highest civil service levels.

``We are in the beginning phase of a plan to downsize headquarters significantly,'' said agency spokesman Brian Welch. The order, issued Friday by administrator Daniel Goldin, bars transfer to headquarters from NASA centers around the country but not moves from center to center.

Testifying before the House space subcommittee, Goldin said he learned on Jan. 12 that President Clinton wanted NASA to make do with smaller annual budgets in the next five years.

``When we lost the money, I was in shock,'' said Goldin. He said NASA had already cut its spending by 30 percent.

NASA has given veteran employees two opportunities in recent years to take a lump sum for early retirements. In the first buyout, 1,178 employees accepted; in the second, Goldin said, 657 signed up.

The agency went from 25,000 employees a few years ago to 22,412 as of last December 1. At its height, during the Apollo moon program, NASA had 33,000 on its payroll.

Goldin told the space subcommittee, which draws up NASA's financial authorizations, that he would have final plans for downsizing the agency by May 17. But Rep. F. James Sensenbrenner, R-Wis., the subcommittee chairman, said the numbers were needed by early next month.

Under the budget sent to Capitol Hill last week by President Clinton, the space station would receive $2.1 billion next year. Construction of the station is scheduled to begin in November 1997 when a Russian propulsion and guidance unit is launched aboard a Russian rocket.

American officials are worried that its partners in the European Space Agency may not be able to meet their commitments.

``If the Europeans want to participate, they better make up their minds quickly,'' Sensenbrenner said. ``Lack of European decision is not going to slow down the United States and Russia.''

The Europeans are signed up to provide a pressurized laboratory module called Columbus. In return, some of the elements for the station will be carried aloft on ESA-made Ariane rockets.

``The Europeans should not think the U.S. and Russia will assemble the station with Ariane 5s if the Europeans do not participate with the Columbus module,'' Sensenbrenner said.

Likewise, with Russia, he said.

``If relations between the United States and Russia get worse,'' he said, ``The United States will maintain its independent capability to continue on with the space station. I have a letter from President Clinton that effect. That was the price for my support for the space station last year when it came up.''