The Prince George's County branch of the NAACP announced its opposition yesterday to a proposal to accelerate the county's magnet school desegregation plan, arguing that officials have not evaluated thoroughly whether the six-month-old program is working.

The civil rights organization said it wants more information about whether black children are bearing an unfair burden in busing patterns and school closings, and whether they are benefiting less than white children from new magnet programs.

The concerns outlined by the NAACP, a plaintiff in a 13-year-old desegregation suit against the county, echo recent criticism by Prince George's County Council members that school administrators are moving too fast in implementing the ambitious magnet plan.

"We don't know what the results of the first year of the program have been," said Richard (Steve) Brown, executive director of the county NAACP. "We don't know whether that did work or didn't work."

The County Board of Education is scheduled to vote on the plan Thursday.

This is the first time the NAACP has registered such strong opposition to the plans of school officials. Brown said the group's position reflected a unanimous vote taken last week by the branch's board of directors.

The group's rejection of the county's proposed timetable, however, does not necessarily scuttle the plan. Attorneys for the NAACP said yesterday that, although they share some of the concerns of the local branch, they have received no orders from the national organization to file a legal objection to the magnet plan.

Some of the other plaintiffs in the desegregation suit said they do not agree with the local NAACP's position.

"For 13 years school officials have . . . dragged their feet. Now I think we should move forward," said Thomas A. Newman Jr., one of several plaintiffs in the class action suit.

Brown called upon Superintendent John A. Murphy to produce a "thorough evaluation" of the first phase of the plan -- a system of a dozen magnet schools -- and to proceed more slowly with the remaining phases. Last month, Murphy proposed speeding up implementation of the desegregation plan, saying he would establish 20 new magnet school programs by the 1987-88 school year.

"The acceleration in and of itself is not inherently bad, but the acceleration appears to visit a great majority of the busing on black kids and the black community," said Alvin Thornton, a member of the NAACP and a citizens committee appointed to advise school officials on the plan.

Murphy dismissed the NAACP's position as "political," referring to Brown's recent announcement that he is running this year for the Maryland Senate.

"They have made the decision that no action should be taken. If the schools are not integrated, they should accept full responsibility," Murphy said.

School officials said that the NAACP's statement contradicted a position taken last year when the group urged the county to speed up the original five-year timetable outlined for the magnet program.

The plan, designed to draw white students to predominantly black schools, was implemented after a dozen years of busing failed to improve racial balance in the county's 175 schools. Details were laid out in a legal agreement signed by the NAACP and the school board.

Yesterday's statement follows similar criticism from the County Council, issued last week in a letter asking the school board to review the program before taking further action.