Secretary of State George P. Shultz, amid growing signs of U.S. reluctance to assume a renewed mediation role in Lebanon, said today there is "a long way to go" before international arrangements can made for an Israeli withdrawal from southern Lebanon.

Shultz's remarks, the first public report on the results of a nine-day exploratory mission by Assistant Secretary of State Richard W. Murphy, came at a luncheon meeting with Arab diplomats.

Other State Department officials indicated that the United States is not likely to undertake active mediation unless there is a narrowing of the "major differences" between the governments of Israel, Syria and Lebanon that became obvious by the time Murphy ended his trip last weekend.

One State Department official said "complications, difficulties . . . positions far apart" between the various parties had surfaced over the arrangements Israel is seeking in connection with its withdrawal.

The official said the differences involve such issues as the role of the Israeli-backed South Lebanon militia, the role of U.N. peace-keeping forces and the question of who would occupy what positions following an Israeli withdrawal.

Israeli Foreign Minister Yitzhak Shamir, who more than a week ago first suggested publicly that the United States might serve as a go-between, especially between Israel and Syria, said after meeting Shultz today that he has not yet requested such U.S. mediation.

Shamir seemed less enthusiastic now than he was quoted as being earlier about potential U.S. efforts. He repeated, however, that Israel would like to withdraw its forces from southern Lebanon "as soon as possible, on condition that we have sufficient security arrangements on our northern border."

More discussion of withdrawal is expected when Shultz meets Lebanese Prime Minister Rashid Karami here Tuesday, Syrian Foreign Minister Farouk Charaa later this week and Israeli Prime Minister Shimon Peres in Washington next week.

Shultz, in his remarks to foreign ministers of the Gulf Cooperation Council, an organization of Arab nations in the Persian Gulf, said, "There is now no question about the fact that Israel wants to withdraw as promptly as it can and that it makes that withdrawal not contingent on Syrian withdrawal, as at one time had been the case."

Israel's insistence last year on linking its pullout from Lebanon to that of Syria was one of the unsurmountable roadblocks that stymied Shultz's earlier negotiating efforts in Lebanon.

The failure of that effort became final with the Lebanese repudiation of the May 17, 1983, Israeli-Lebanese agreement.

That bitter experience is believed to be one of the reasons for Shultz's lack of enthusiasm about undertaking another major U.S. role.

Shultz also told the Arab diplomatic luncheon that Israel, Syria and Lebanon are "talking in terms of an expanded . . . mandate" for UNIFIL, the 5,700-man U.N. peace-keeping force in Lebanon, "although just what that means and what role it would play is part of the problem here."

An Israeli diplomat told reporters following Shamir's meeting with Shultz that Israel sees "a certain role" for the U.N. force "on top of" other security arrangements in southern Lebanon, not as a substitute for other arrangements.

The Israeli official said "it is obvious" there will have to be a role for the Israeli-backed south Lebanon force headed by Gen. Antoine Lahad because "as of today, as a combat force there is no alternative."

However, the Lebanese government has insisted that Lahad's force be disbanded or integrated into the Lebanese Army.

Arab diplomats here said this is now the central issue of disagreement with Israel.

State Department spokesman Alan Romberg denied reports from the Middle East that, during his recent mission, Murphy presented a U.S. plan to facilitate the Israeli withdrawal.

"The United States has presented no plan and is not acting as an intermediary," Romberg said, adding that "it remains to be seen if the United States can play a role."