Philip Caryl Jessup, 89, an authority on international law who had been a representative of this country at the United Nations and had served as a member of the International Court of Justice at The Hague from 1960 to 1969, died Jan. 31 at his home in Newtown, Pa. He had Parkinson's disease.

From 1949 to 1953, he represented the United States at the U.N., held the rank of ambassador at large and was one of the closest advisers of then-Secretary of State Dean Acheson.

During his years at the U.N., he played a crucial role in the diplomatic effort to end the 1948-1949 Berlin blockade. With surface access to Berlin cut off by Soviet-controlled forces, the western allies resorted to the famous "Berlin Airlift" to deliver vital supplies to West Berlin.

Among the factors bringing about the blockade was the use of the new West German currency in West Berlin. Dr. Jessup was told to approach a Soviet U.N. delegate when Washington noted that statements by Soviet leader Josef Stalin suddenly were omitting reference to the "currency" issue. Dr. Jessup approached Yakov Malik at a U.N. bar and attempted to find out if these omissions indicated a new Soviet willingness to negotiate. Malik's eventual answer was "yes." Further inquiries resulted in the negotiations that ended the crisis.

Dr. Jessup was a native of New York City and a graduate of Hamilton College. He earned a law degree at Yale University, and a master's degree and a doctorate in international law at Columbia University. He served with the Army in Europe during World War I. He was a member of the faculty of Columbia from 1925 to 1961, where he taught international law and diplomacy.

He was nominated for a judeship on the International Court of Justice at The Hague, better known as the World Court, in 1960. An editorial in The Washington Post greeted his nomination as "in keeping with the best tradition of that institution. Dr. Jessup is an expert on international law and an experienced statesman. He was a member of the United States delegation which helped to shape the charter of the United Nations at San Francisco in 1945, and for five years represented this country in the U.N. Security Council and General Assembly."

During his years on the bench, he was probably best known for his blistering 1966 dissent dealing with apartheid. Two black African nations failed in their attempt to have apartheid declared illegal in what is now Namibia, a nation then occupied by South Africa.

Dr. Jessup was the author of "A Modern Law of Nations," and of a two-volume biography, "Elihu Root," about the American statesman and a founder of the first World Court. Dr. Jessup had served as a trustee of the Council on Foreign Relations, the Foreign Policy Association and the Carnegie Endowment for International Peace.

Survivors include his wife Lois; a son, Philip C. Jr., of Washington; a brother, Richard S., of Hightstown, N.J.; three grandchildren, and four stepgrandchildren.