Egypt, Israel and the United States today completed months of negotiations for the establishment of a multinational peacekeeping force to police the Sinai following the withdrawl next April of Israeli forces from the last occupied Egyptian territory there.

The tentative agreement reached by the negotiators, which must be approved by the Israeli and Egyptian governments, outlines a major role for the United States in assuring a lasting peace between the two Middle East nations, including the permanent stationing of U.S. troops in the Middle East for the first time.

Deputy Assistant Secretary of State Michael Sterner, who let the U.S. delegation to the talks, said at a press conference that one battalion of 700 to 800 U.S. troops will be included in the 2,000-man multinational force.

In addition, the United States is to provide the "director general" in over-all charge of the force, a logistics unit, much of the funding and a team of about 70 civilian observers who will be stationed on both sides of the Israeli-Egyptian border, Sterner said.

"We have been delighted to play a constructive role in bringing about this agreement," Sterner said. "We think this is a very good development, one that will strengthen the peace that exists in the area and the future of the peace process."

Sterner refused to say which other governments have agreed to provide troops but he indicated that some of the 12 to 15 contacted out of a list of 30 potential contributors had already given a "firm commitment" to do so.

Among the nations mentioned in press reports as possible participants have been Canada, Australia, New Zealand, Urguay and Fiji. The issue has stirred a controversy in Australia, however, and other nations have been reluctant to commit themselves.

Sterner said he expected the force to be in place around mid-March of next year, about a month before the deadline set for the final Israeli withdrawl from the Sinai.

Under the Camp David accords, which laid the groundwork for an Egyptian-Israeli peace treaty and Israel's return of the Sinai to Egypt, the United States promised to arrange a suitable "multinational force" to police the area after Israli withdrawl from the territory it captured in the 1967 war. Israel insisted on such an approach rather than the customary peacekeeping forces under the United Nations, whose membership is largely hostile to Israel.

Sterner said he did not believe the Israeli raid on the Iraqi nuclear reactor near Baghdad had had any "material impact" on the negotiations because, he said, both sides had made it clear that they were an essential part of the peace process that had to continue.

Egyptian President Anwar Sadat has sharply criticized the raid and ordered several economic and cultural exchange programs with Israel frozen to show displeasure. But otherwise he has not allowed it to affect the peace process.

Sterner said he expected the agreement on establishing the Sinai force to be initialed "in the very near future," indicating that it would be after the June 30 Israeli elections. But he said it was not clear how soon the two countries would hold a singing ceremony because of the need to have governments and parliaments in both countries approve it.

He said the U.S. Senate would not be called upon to approve the agreement because "this is not a treaty" but that Congress would have to authorize the participation of U.S. troops in the force as well as the funds to pay one-third or more of the cost.

Under the accord, the three nations have agreed to split three ways the cost of the peacekeeping operation, which Sterner said would be "probably below $100 million" annually. But the United States has committed itself, he said, to picking up a disporoportionate share of the starting-up costs which he estimated at "something like $200 million."

He said the three nations hoped that any "affluent" country willing to send a contingent to participate in the peacekeeping force might also be willing to contribute money or at least provide troops free of charge.

As described by Sterner, this is how the peacekeeping force will be constituted and operate: It will have three lightly armed infantry battalions, two in the north stationed at Rafah and one in the south at Sharm el-Sheikh, with its total size not exceeding 2,000 troops. There will be additional support units, such as aviation and logistics, raising the overall troop total to between 2,000 and 3,000.

The force will have a civilian U.S. director general in overall charge but a non-American field commanmder chosen by the director with the approval of both Israel and Egypt. Each contingent will have its own national commander.

There will also be a U.S. team of 70 observers who Sterner indicated would probably be drawn from the American Sinai Field Mission now operating as a temporary early warning system between Egyptian and Israeli lines. That mission has an authorized limit of 200 unarmed civilians but has been functioning with fewer.

Sterner said he assumed the Sinai field mission, staffed by the Dallas-based E-Systems Company, would be dissolved once the new force is established.

The force's director general, Sterner said, will be authorized to deploy his troops in any matter he sees fit, including reconnaissance patrols, within the limits stipulated by the Camp David treaty and the agreement. Whether the force is authorized to use arms was not clear he said.

Sterner called the new agreement an "open-ended package," by which he meant that it contains no date for terminating the new peacekeeping force.