A group of black Foreign Service officers said yesterday that they have filed a class-action suit in U.S. District Court here charging the State Department with systematic racial discrimination.

George Chuzi, an attorney for the group, told a news conference that an example of such discrimination was a cable, allegedly issued by former national security adviser Zbigniew Brzezinski, "precluding the assignment of black FSOs [Foreign Service officers] to southern Africa." He did not elaborate.

Brzezinski, reached in Florida through his office, said, "No such cable was authorized by me and I have no knowledge of it."

The suit, filed Oct. 17, asks retroactive pay, promotions and assignments for the 257 black FSOs included as members of the class. It charges that black FSOs have suffered from discriminatory evaluations, lack of promotions, undesirable assignments and unfavorable treatment by career counselors.

"Written policies and procedures are not followed in regard to blacks. They arbitrarily break their own rules," said Walter Thomas, one of the plaintiffs in the suit. He was "selected out" of the Foreign Service in 1984 after a seven-year career as an administrative officer in Brazil, Ghana and Antigua.

According to the suit, Thomas "was supervised by white FSOs and rated less favorably than his white peers, although his performance was the same as or better than the whites. " He was considered "a minority troublemaker," was never promoted and was expelled from the service in retaliation for his complaint to the Equal Employment Opportunity Commission, the suit alleges.

Bernard Johns, another plaintiff, who has been an FSO for 18 years, told reporters that he was chided for studying Arabic and for questioning U.S. policy in the Middle East. "The prevailing attitude is that blacks are not effective in the Middle East," he said. "The comment was made that I had no business studying Arabic." He is now the only black American diplomat who speaks Arabic, he said, but has been denied a Mideast assignment for the past two years.

He said the office assigned to him as a junior political officer in Libya was "in an empty wing, isolated from the rest of the embassy," and that he felt "a residual and pervasive resentment" of his presence.

"You find yourself enduring the slights . . . and when you complain about it you're labeled a troublemaker," he said.

State Department spokesman Charles E. Redman declined to comment on the suit, but said minority issues are "a matter of serious concern" at State. "Major changes in department procedures designed to improve the career opportunities of minorities are in the process of being implemented . . . in cooperation with minority officers themselves who have identified some areas of concern," he said.

Secretary of State George P. Shultz sent a cable Sept. 5 to all diplomatic posts expressing concern that minority and female representation in the Foreign Service is "unacceptably low." The cable asked diplomats to stress recruitment of blacks, and promised "exceptional efforts" to counsel, promote and retain black FSOs.

"We cannot tolerate any form of discrimination or accept anything less than a truly representative Foreign Service," it said.

Johns said similar directives have had "very little if any impact . . . for the past 30 years."

More than 100 black current and former FSOs, including several ambassadors, have donated funds to support the lawsuit, which also is backed by the Congressional Black Caucus and the Rev. Jesse L. Jackson's Rainbow Coalition, Johns said.