As President Reagan prepares to meet today with an Arab League delegation headed by King Hassan of Morocco, a senior State Department official said yesterday the Arab countries must "come out of the closet" and make clear whether they are willing to recognize Israel.

The official, who spoke with reporters on condition he not be identified, was referring to what he called "constructive elements" in the declaration adopted last month by Arab states at Fez, Morocco, as their plan for resolving the Arab-Israeli conflict.

The Hassan delegation's purpose is to explain the Fez plan to Reagan and to discuss the U.S. peace initiative outlined by the president on Sept. 1. The administration has been careful to describe today's White House meeting as an exchange of views rather than negotiations, but the senior official stressed that the United States intends to press the Arabs to be more explicit about such aspects of the Fez plan as a provision implying recognition of Israel's right to exist.

Referring to this implication, contained in the seventh point of the Fez declaration, the official said: "Does point seven mean what we think? If so, why not say so?"

According to the official, the "key element" that Reagan will stress to the six-nation Arab delegation is that "negotiations for peace must take place around a table between Arabs and Israelis and Americans, not between Americans and Arabs or Americans and Israelis. There is no alternative to that."

"We do not expect a dramatic breakthrough, but rather a thoughtful discussion," the official said.

But he added that the United States is hopeful that the meeting will give momentum to the effort to persuade Jordan's King Hussein to join Egypt, Israel and the United States in negotiating an interim autonomy agreement for the Palestinian inhabitants of the Israeli-occupied West Bank and Gaza Strip.

The core of the Reagan initiative calls for these territories to gain eventual independence "in association with Jordan." The Fez declaration, however, reaffirms the traditional Arab position that the territories must become an independent Palestinian state under the leadership of the Palestine Liberation Organization.

U.S. policymakers nevertheless believe that Hussein will be prepared to join the talks under the right conditions.

Asked what Hussein needs to come forward, the official, contending that the king already has substantial backing from Palestinians in the occupied territories, said Hussein also requires "support from a substantial number of moderate Arab countries led by Saudi Arabia."

In addition to Hassan of Morocco, the delegation includes the foreign ministers of Jordan, Saudi Arabia, Syria, Algeria and Tunisia and the secretary general of the Arab League.

A PLO representative also is attached to the group, but U.S. officials have made clear that he will not be received and that there will be no departure from U.S. policy of refusing to deal with the PLO until it accepts Israel's right to exist.

While insisting that Reagan remains committed to the long-range goals of his initiative, the official said the intention at this time is to breathe new life into the negotiations on a temporary autonomy agreement and thus pave the way for a final settlement later. "We hope the Arab group will leave sensitized to the idea that we are talking about a two-step process, and be more willing to enter the first step," he said.

Secretary of State George P. Shultz met yesterday with Israeli Foreign Minister Yitzhak Shamir for the third time in a week to discuss the other major Mideast problem preoccupying the United States -- the withdrawal of foreign forces from Lebanon.

Sources on both sides said the meeting was primarily to inform Shamir about Reagan's meeting Tuesday with Lebanese President Amin Gemayel.

Gemayel raised with Reagan the possibility of an expanded role for the multinational force in Beirut, which contains 1,200 American Marines. Defense Secretary Caspar W. Weinberger said yesterday that he does not anticipate an increase in the size of the U.S. force or a broadening of its mission.

Some U.S. officials privately have suggested that the multinational force might eventually be assigned other duties, such as monitoring traffic on the strategic Beirut-Damascus highway.

But they also have stressed that negotiations on withdrawal of foreign forces will have to proceed much further before the questions of an expanded or changed role for the U.S. forces can be addressed.