The Metropolitan Washington Council of the AFL-CIO sent letters yesterday to board members of the Kennedy Center stating that the labor contract between the center and the Opera House orchestra would expire a week from Sunday.

In a phone interview, Joslyn N. Williams, the council's president, said that such notification is standard in anticipation of a contract expiration. He also said that the orchestra union requested several weeks ago that the letters be sent.

In the letter, Williams told the board members, "We are optimistic that an equitable settlement can be worked out. However, the musicians have notified us of their intent to strike should this not be the case."

Allen G. Siegel, the labor counsel for the center, said in a phone interview, "I don't think either side wants a strike." Siegel said that there have been only two bargaining sessions, and he described them as "very civilized, and at times more than that."

Among the issues still to be resolved, Williams wrote, are salaries and fringe benefits. "The Opera House musicians are grossly underpaid in general as well as relative to other orchestras in similar sized cities. They have few fringe benefits which most other employees enjoy, such as health benefits. They are also experiencing a decline in working hours due to insufficient numbers of productions at the Opera House."

Williams added that "the musicians have developed a creative proposal for expanding the operations of the Opera House."

Siegel said that he has had discussions with Board Chairman Roger L. Stevens and Artistic Administrator Marta Istomin on the possibilities of new programs in the Opera House.

Last year, unions at the center accepted a wage freeze for this season because of the center's financial problems. Siegel said that the musicians agreed to only a year's contract, however, while others accepted three-year agreements.

The negotiations also cover the orchestra for the Terrace Theater.