Federal mediators have called a meeting for tonight between representatives of Boston University's trustees and faculty in hopes of settling the six-day-old faculty strike there.

Gerard M. Gomez, a mediator in the Boston branch of the Federal Mediation and Conciliation Service, said yesterday he called the meeting because negotiators for B.U.'s trustees and faculty have reached an impasse he hopes can be overcome by impartial mediation.

Gomez, who will be assisted by fellow-mediator James Arthur, declined to comment on his role in the talks or on possible success of the negotiations. He said further meetings will be called if necessary.

The mediators said they will not attempt to settle the dispute between the university and its striking librarians and clerical workers, who have voted not to return to work until B.U. recognizes their unions.

"We welcome this step and we're looking forward to the meeting." Robert C. Bergenheim, a university trustee, said yesterday. "Under pressure from mediators we should be able to wrap the matter up," he added.

The trustees have delayed ratification of the three-year contract, pending clarification of sections they have called ambiguous. The faculty and the university negotiators have already ratified the contract.

These sections include the date on which the contract goes into effect and expires, the extent of the college deans' authority, the method of determining salary increases and a no-strike clause.

Michael McKeon, a spokesman for the union, said the faculty looks forward to meeting with federal mediators. "Hopefully this will enable us to have more useful discussions with the trustees," he said.

B.U. public relations officers refused yesterday to discuss the federal mediation.

Bergenheim warned the faculty Monday that unless they end the strike by Saturday, the trustees will rescind their informal approval of sections of the contract already agreed upon, including a 32.4-per-cent salary increase for the faculty over three years.

"We don't want to do this, but a continued strike would be very costly," Bergenheim said yesterday.

McKeon said, "We've made substantial concessions and now it's time for the administration to make concessions.