The services performed by the United Nations in the Middle East have rarely been appreciated, and this is nowhere more evident than in the conference of the Mideast powers, together with the Soviet Union and the United States, called by Secretary General Kurt Waldheim. It is being dismissed as no more than an attempt to get into the act.

In fact, it is a deeply sincere effort to go beyond the limitations of the Cairo conference called by President Anwar Sadat of Egypt. The outcome in Cairo is chancy. At one extreme it could end with a separate peace between Egypt and Israel that would be no guarantee against future turmoil. Or it could end in breakdown with Sadat's position gravely weakened.

Waldeim's hope is to get the Arab front-line states, Syria and Jordon, to sit down with all the other participants for a thorough, unspectacular discussion of the whole tortured delimma of the Middle east. This has been made more difficult by Sadat's impulsive action in breaking off relations with all nations involved, whether directly or indirectly in the anti-Cairo session in Libya.

"I see my role as that of mediator who must try to be helpful in a practical way," Waldheim said in a talk with this reporter in his office on the 38th floor of the U.N. building. "I hope it will be possible for all the parties to return to the conference table with an important impact on the negotiating process. It is not possible, in my opinion, to go directly from Cairo to a Geneva conference. That is why I am proposing what is, in effect, a safety net where all the differences can be examined."

Sadat's immediate respose was that he would be represented at the U.N. conference. Israel declined. The United States accepted. Waldheim has been in almost daily contact with Secretary of State Cyrus Vance. He believes that Vance has made a wise decision in his mission to the Middle East to talk with all the contending parties.

Jordan had indicated a willingness to attend the U.N. session and Waldheim does not rule out the possibility that President Hafez Assad of Syria will in the end agree to representation. Nor does he preclude the possibility that the Soviet Union will participate, even though Moscow boycotted the Cairo conference and directed an attack on Sadat's pilgrimmage to Jerusalem.

In designating Lt. Gen. Ensio Siilasvuo to be the U.N. representative at the Cairo conference, Waldheim underscored the importance of the United Nation's role in the area. Siilasvuo directed the difficult negotiations a t Kilometer 101 that finally wound up the Israeli-Egyption Sinai disengagement. With the eyes of the world on the tent that housed the negotiators of the two sides, Siilasvuo was widely recognized as having done a masterful job in helping to nail down the demarcation lines of a highly complex agreement.

He is chief coordinator of the U.N. peace-keeping forces in the Middle East. It is here that the contribution of the United Nations has been invaluable. Repeated U.N. peace-keeping foreces recruited from smaller member states have policed truce lines.

Such a force, composed of troops from four nations, currently patrols the dividing line between Israel and Syria in the Golan Heights area. To see these men with their blue U.N. insignia keeping watch on the truce line is to believe that a world organization can fulfill the hopes raised so high after the United Nations was founded in 1945.

If out of the present pause further Israeli concessions should come, additional U.N. forces would be called into being. More remotely, should the holy places in Jerusalem be made accessible to all peoples of all creeds, the access routes would be maintained by a bluehelmeted U.N. force. In ensuring the agreed neutrality of specific sectors, forces of the superpowers can have no role, Waldheim says, as he gives overall supervision to U.N. military operations.

He is under no illusions about the seriousness of the deep split in the would body between the developed and the underdeveloped nations. The failure of the North-South dialogue last summer to agree on a common approach on commodity prices and on a common fund to begin to offset the grave imbalance between the haves and the havenots was a major blow.

"It must be remembered that the peoples in the developing nations are three-fourths of the world populations against one-fouth in the comparatively prosperous developed countries. If this great gap continues to exist, poverty will increase and political instability is bound to follow.

But Waldheim is an optimist about the ultimate outcome when the "have" nations come to a realization that a common fund for rehabilitation is issential. For a Secretary General - he is now in his second term - you have to be an optimist to go on.