The union representing nurses at Prince George's County's three public health care facilities has filed suit in federal court in Baltimore seeking to rescind the dismissal last week of about 650 employes at the facilities.

Representatives of the union and the nonprofit Community Hospital and Health Care Systems Inc., which operates the facilities, met for about six hours yesterday on orders of U.S. District Judge Frank A. Kaufman, but were unable to reach an agreement on the overall dispute, a union attorney said.

However, a court source said the hospital system has agreed there will be no additional dismissals pending a hearing at 6 p.m. Monday on the union's request for a temporary restraining order and injunction.

Larry Grosser, attorney for the nurses union, said there have been reports that another round of dismissals is being considered and could take place as soon as October.

A spokeswoman said last night that officials of the nonprofit corporation and the HCA Management Co., which was hired in July to manage the facilities -- Prince George's General and Greater Laurel-Beltsville hospitals and the Bowie Health Care Center -- would not comment on the litigation or meetings.

Attorneys for the two sides are to meet again today and report their results to Kaufman.

The suit, filed by the Prince George's chapter of the Maryland Nurses Association against the hospital corporation, contends that the dismissals were in violation of patient care standards that are a part of the union's master work agreement. The agreement, reached last year, is in effect through November 1986.

The union's suit asks that the status of the system's operation be returned to what it was before the dismissals to allow the two sides to determine staffing necessary to meet standards specified in the union contract.

Although only 50 registered nurses were among those fired, Grosser said the dismissal of the other employes is "raising havoc with patient care. Nurses are having to do what those 650 people had to do and nurses are not spending enough time at the bedside."

"The nurses' contract indicates that a certain level of patient care must be maintained," Grosser said. "We say that the level of care can't be maintained" with the staff reductions.

Hospital corporation and management company officials said the mass firing -- announced Aug. 27 and carried out last Saturday -- was necessary to make the financially troubled system more efficient. Officials estimated the staff cuts would save the hospital system $3 million, roughly the amount of its operating deficit last year.

Mary Stuart Rizk, spokeswoman for the hospital system, said last night that "the segmentation of the reduction was very sensitive to maintaining quality care."

She said the staff changes made possible "primary nursing care, which enhances the quality of care and does not detract from it. Thirty-one RNs in patient care . . . and 19 managerial RNs were affected among 651 persons laid off," Rizk said.

"The scope of the impact on patient care was minimal," she said.

"This was a financially motivated decision to ensure that the facilities remain to offer care to the community."

Rizk said the reductions were in line with the number of patients being served by the three facilities. According to statistics released by the hospital system, occupancy of the 550 beds at Prince George's General has declined from 75 percent in 1982 to 69 percent in 1985 and the average patient's stay has dropped from 8.6 days to 7.6 days during the same period.

At Greater Laurel-Beltsville, the occupancy rate has remained about 51 percent, but the average stay has declined from 8.4 days to 7.2 days.

Before the dismissals, the three facilities employed a total of 2,824 persons, 700 of them nurses.