Zbigniew Brzezinski, White House national security adviser to President Carter, said Wednesday that "I think the time has come for the United States to talk to the Palestine Liberation Organization."

Brzezinski, at a breakfast meeting with reporters, said the purpose of the discussions should be to encourage the PLO to be more moderate and to become engaged in the peace process in the Middle East.

The former official said it would be "premature" to have formal U.S.-PLO negotiations, which are barred under a pledge to Israel in 1975 by then-Secretary of State Henry A. Kissinger. While advocating discussions that fall short of the "negotiations" specified in the 1975 commitment, Brzezinski said he did not believe a pledge of that sort by a secretary of state is "eternally binding."

"We should move away from the covert, embarrassed discussions we have been having with them," Brzezinski said of the PLO. He declined to specify the discussions he referred to.

Egyptian President Anwar Sadat last week urged President Reagan to undertake U.S. contacts with the PLO for the same reason cited by Brzezinski. Secretary of State Alexander M. Haig Jr. said the Reagan administration is sticking to the 1975 commitment, which has been interpreted by successive U.S. administrations as a ban on all contacts with the PLO unless that organization recognizes Israel and U.N. resolutions on the Middle East.

As late as Tuesday, Haig said in New Orleans that the U.S. commitment to Israel regarding the PLO is "firm" and that "I see no possibility of it being modified in the days ahead."