A federal judge ruled yesterday that the D.C. Fire Department can begin hiring immediately under an affirmative action plan that reserves almost two-thirds of new jobs for blacks.

U.S. District Court Judge Charles R. Richey said, however, that no promotions may be made under the plan until the Court of Appeals takes up the issue.

The order, issued early yesterday evening, follows a complicated round of legal maneuvering in which lawyers for both the federal government and a group of black firefighters urged that the hiring plan be shelved.

But Richey noted that in his last major order on April 1 he had approved the hiring plan as "minimally acceptable" as an effort to overcome past discrimination, although he ruled the promotion plan invalid because its quotas "unnecessarily trammel" the rights of white firefighters.

Yesterday, Richey said the District government must be able to hire new firefighters quickly because its fire department is "understaffed and overworked, a situation which threatens the health and safety" of D.C. residents.

City officials told the judge that the department has 127 vacancies in its authorized strength of 1,374 and already has spent $2.5 million for overtime this fiscal year.

Although he earlier had directed the city to submit a new hiring plan by May 15 that would consider "viable alternatives to race-conscious numerical goals or quotas," Richey said the District could begin using the plan now, as city attorneys had requested, because there is "substantial likelihood" that he will approve it again after fuller arguments.

The plan specifies that the racial composition of the fire department should reflect the proportion of blacks in the working-age population of the city, which is about two-thirds black. Blacks now account for 38 percent of D.C. firefighters.

Attorneys for the federal government said the fire department should do all its hiring as well as its promotions without racial preferences or discrimination.

In papers filed Thursday the Justice Department suggested that instead of keeping separate lists of blacks and whites based on scores on its current written entrance exam, as the hiring plan provides, the D.C. Fire Department should set a minimum standard of mental ability for all applicants and then choose those who do best on a "job-related physical abilities test."

Joan A. Burt, attorney for the group of black firefighters who originally filed suit against the District government, also opposed the affirmative action plan. She said the fire department has not yet taken steps, ordered two years ago by a hearing examiner, to develop a new entrance exam that would be fair and properly job related.

Another party in the case, Local 36 of the International Association of Fire Fighters, the union that represents all D.C. firefighters, had only attacked the promotion part of the plan.

Last week, the D.C. government appealed Richey's entire April 1 order to the U.S. Court of Appeals. The judge said yesterday that under court rules that made it impossible for him to consider any proposed revisions to the promotion parts of the plan and probably delayed a final resolution of the case.

"The court finds itself frustrated" by the city's appeal, Richey declared, adding that the move has "hamstrung the diligent efforts of all the parties . . . to expeditiously proceed."

Richey's new order, which the judge said will stay in effect "until or unless" a higher court changes it, can also be appealed. There was no indication last night whether any of the parties would do so.