Contract negotiations between striking nurses and the management of Group Health Association were at an impasse yesterday, and a federal mediator said he would not reschedule talks until there is some sign of movement.

With the strike now beginning its third week, the two sides have entered "a time of testing" each other, according to Brian Flores, one of the two federal mediators handling the negotiations.

"Group Health hasn't made any announcement that they're replacing anyone and no nurses have gone back to work," said Flores. After adjourning the talks just before midnight Tuesday, Flores said that by last evening neither side had altered its position.

"Somebody's going to have to blink," he said.

Statements issued by GHA, which has 112,000 member-patients and is the largest health maintenance organization in the Washington metropolitan area, and by the 85-member Registered Nurses and Physical Therapists Association, indicated that the key issue separating them is the level of a pay increase for next year.

Proposed annual pay levels for this year are within a few hundred dollars of each other. Where the two diverge is in the middle of 1983: GHA has offered the nurses a starting minimum of $18,360, to begin Jan. 1 and to remain in effect throughout the year. The nurses proposed $18,918 at the start of the year and a second increment to $20,088 starting July 1.

According to a source involved in the negotiations on behalf of the nurses, the $20,088 figure is of paramount importance because it would become the base for all other salaries. "Only two of our members are at the starting level, so it's crucial that we have a high base salary figure to build on," the source said.

If the nurses were to win this top minimum, those at the high end of the scale, with 10 years of experience, would be earning $25,515 by July of 1983. Under GHA's proposal, the top scale would be $23,499.

In a statement distributed to the strikers yesterday, GHA executive director Dr. Edward J. Hinman said: "In contrast to the GHA proposal, your bargaining committee has continued to press unrealistic salary demands . . . . Given the current state of the economy and the fact that all salary increases must be passed on to GHA's members through premium increases, GHA simply cannot meet the union's demands."

Other unresolved issues are the nurses' demands for improved staffing conditions and questions about possible future binding arbitration of contract disputes.