President Anwar Sadat dismissed the dispute over international forces for the Sinai as a "side issue" today and predicted that the United States, Israel and Egypt will swiftly work it out.

Sadat's effort to play down the disagreement underlined his determination to let nothing stand in the way of his peace treaty with Israel and the return of Egypt's Sinai Peninsula. It contrasted sharply with the irriation reported in Washington over Irsael's rejection of a U.S.-Soviet plan and demonstrated once again Sadat's sweeping approach to diplomacy.

"Egypt, Irsael and the United States shall reach agreement in the very near future on this issue," Sadat told reporters in Alexandria after discussions with the visiting Irsael opposition leader, Shimon Peres. "I don't see it as a matter of difference and it should noit be. It is a side issue on which we shall reach agreement."

The Security Council mandate for the United Nations Emergency Force that has been patrolling in the Sinai since after the 1973 war ran out today, in principle leaving Irsaeli and Egyptian lines in many parts of the Sinai without any internationally sanctioned force to separate them.

The 4,000-man UNEF force, however, is expected to remain in place for weeks without the mandate, officially winding up its operations and in practice separating Egyptian and Israeli forces until a substitute can be found.

Egyptian officials have said the mandate expiration would not interfere with a ceremony planned Wednesday in the Sinai to mark the return of a 2,000-square-mile patch of desert that constitutes the second phase of the Israeli withdrawal agreed in the March 26 treaty.

Even with Sadat's assurances of a solution to the forces issue, the United States still must persuade Israel to drop its objections to the U.S.-Soviet plan or finding another compromise solution to which the Soviet Union will agree.

Washington had proposed, with Moscow's accord, that UNEF be replaced by a strengthened version of the United Nations Truce Supervisory Organization, an unarmed international force of about 150 men that has been in the area since the 1948 war in which Israel won its independence.

Egypt accepted this proposal but Israel rejected it, insisting on an armed force and one that reports to the Security Council rather than to the secretary general as do the truce supervisors.

Egyptian officials, while insisting that the United Nations must remain involved in policing the withdrawal lines, were unusually cautions in reacting to the Israel refusal.

The government-guided press reported most of the story from Washington and the United Nations, strengthening the impression among observers here that the Egyptian government regards the problem as something Washington will have to deal with. The Defense Ministry announced, however, that Egyptian Defense Minister Kamal Hassan Ali will discuss the impasse with his Israeli counterpart, Ezer Weizman, in military talks scheduled late this month in the northern Israeli port of Haifa.

Sadat has been going out of his way to avoid the appearance of disagreement with Israel, and in particular with Prime Minister Menachem Begin. The two leaders were reported to have got along smoothly at their recent summit conference in Alexandria, but only at the price of avoiding real bargaining on the tough issues separating their countries.