With Secretary of State Alexander M. Haig Jr. serving as official witness, the ambassadors of Egypt and Israel yesterday signed the agreement that establishes a multinational peace-keeping force to patrol the Sinai after Israeli troops are withdrawn next April.

The United States has agreed to supply slightly more than half of the 2,000-man force, marking the first time that U.S. troops will be permanently stationed in the Middle East.

But the identity of the other participants in the multinational force remained largely unknown yesterday even as the agreement was being signed at a State Department ceremony.

The United States has sounded out a number of countries about contributing to the force, but only Fiji has publicly agreed to do so. State Department spoesman Dean Fischer said that other countries have committed themselves to providing troops to the force but that an announcement of the force's makeup is being delayed until all details are worked out.

Israel occupied the Sinai during its 1967 war with Egypt and agreed to withdraw from the territory as part of the 1979 Egyptian-Israeli peace treaty. As part of the Camp David accords that led to the treaty, the United States pledged to arrange a suitable multinational force to police the area after the Israeli withdrawal.

The United Nations declined to establish such a force, citing the probability of a Soviet veto, so Israel and Egypt agreed in Cairo June 25 to set up the force outside of U.N. auspices.

Haig said during the ceremony that the agreement resulted from "skilled diplomacy" and reflected "a new confidence in the Middle East in America's leadership."

Egyptian Ambassador Ashraf Ghorbal said the U.S. role in helping bring about the agreement as well as the American effort to arrange a cease-fire in Lebanon marked the Reagan administration's "first achievements" in the Middle East.

"His augurs well for the future," he said.

Israeli Ambassador Ephraim Evron also cited the United States as "an active partner" in setting up the force.

"We should all remember that the credibility of an American promise and commitment, on any issue, is essential for keeping the momentum of the Camp David accords," he said.

The United States will contribute one infantry battalion of 700 to 800 men and a logistics unit to the force. In addition, the "director general" in overall charge of the force is to be an American, and the United States has agreed to pay a large share of the force's initial costs.

After these start-up costs, the United States, Egypt and Israel are to share the costs of the force equally.

The signing of the agreement came the day before Egyptian President Anwar Sadat is to arrive in Washington for talks with President Reagan and other top administration officails about the peace process in the Middle Est and the stalled Palestinian autonomy talks.