Zbigniew Brzezinski sought to assure South Korea yesterday that it will not be left out of any plan for peace talks involving the Korean Peninsula.

He said that the United States would not participate in any plan that might "bypass or isolate South Korea."

Brzezinski made is remarks as he left Seoul yesterday to return to the United States, concluding his visits to China, Japan and South Korea.

President Carter's national security affairs adviser had met earlier with South Korean President Park Chung Hee and left with him a private letter from Carter. Its contents were not disclosed.

South Koreans have been jittery in recent months because of recurring rumors in Seoul that some new kind of peace plan affecting the Peninsula is being discussed among world leaders.

They have also been concerned that the United States might undertake private talks with North Korea.

Brzezinski was asked about reports of a proposed three-party peace conference that would include the two Koreas and the United States.Such a plan has been circulated by Romanian President Nicolae Ceausescu and suggested by him to President Carter. Initially it was thought the idea originated in North Korea but observers here and in Seoul now doubt that.

Brzezinski said the basic U.S. position is to support any South Korean effort to "enhance peace and security" on the Korean peninsula, but he added:

"The United States will not be a party to any effort to bypass or isolate South Korea. It is in that context that any specific proposal for any kind of conference has to be seen."

The Korean situation is believed to have been one of the subjects discussed by Brzezinski and Chinese leaders. One of them, Communist Party Chairman Hua Kuo-feng, has just visited Pyongyang , the capital of North Korea.

Brzezinski was asked about reports that he and the Chinese officials had been unable to agree on a formula for peace talks. He refused to comment, saying only that in Peking, "The areas of agreement were more important than areas of disagreemen."

He added, however, that it is "normal" for two major powers to have "different perspectives" on some important issues.

Brzezinski refused to answer questions about the Korean influence buying scandal in Washington or the latest congressional effort to have a former South Korean ambassador, Kim Dong Jo, made available for questioning about his role in alleged bribery of congressmen.

In Washington, the House International Relations Committee has passed a nonbinding resolutin warning South Korea that economic aid will be cut off if Kim's testimony is not made available. President Park's government has refused to have Kim testify on grounds it could violate his diplomatic immunity.

In a statement released before leaving for Washington, Brzezinski said the United States will remain closely associated with South Korea in main-[TEXT OMITTED FROM THE SOURCE]