Prime Minister Shimon Peres of Israel and King Hassan II of Morocco held closed-door exploratory talks today amid mounting diplomatic speculation discounting the likelihood that their surprise meeting would produce a major breakthrough in Middle East peace initiatives.

Neither side had made any substantive official statements about the talks more than 24 hours after Peres unexpectedly flew here on an Israeli Air Force executive jet with aides and reporters for the first summit meeting between Israeli and Arab leaders in five years.

But even before the two days of historic talks started at the mountain resort of Ifrane, Syria broke diplomatic relations with Morocco to protest its prowestern monarch's "treason" in following Egypt's trailblazing nine-year-old lead of dealing openly with Israel.

Governments across the Arab world, from Morocco's neighbor Algeria and ally Libya to normally friendly Iraq, 3,500 miles to the east, echoed Syria's condemnation, although without responding to its entreaty to break relations with Morocco.

With many of Hassan's fellow moderate Arabs remaining silent pending the outcome of the talks, only President Hosni Mubarak of Egypt, which concluded a separate peace with Israel in 1979, openly welcomed the Peres-Hassan initiative.

In Cairo, Mubarak told his ruling National Democratic Party, "I support the meeting with all I have got and I believe the Egyptian people are with me."

Moroccan officials said the first round of talks at Hassan's Prainite Palace at Ifrane, in the Atlas Mountains 100 miles east of this capital, were a purely Israeli-Moroccan affair. Western diplomats noted that the king, who currently heads the 22-member Arab League, has no mandate from it for meeting with Peres. The diplomats said they doubted that Hassan had kept the other Arabs closely briefed on the contacts with Peres that Israeli sources suggested have been under way for several months. There also is no indication that the talks here are linked to Vice President Bush's mission to the Middle East next week.

But following the breakdown of the peace initiative between King Hussein of Jordan and Chairman Yasser Arafat of the Palestine Liberation Organization, Hassan in April staked out a potential negotiating role for himself to break the Arab-Israeli impasse.

In an interview with the Revue Des Deux Mondes in Paris, he urged the Arabs to choose a leader to sound out Peres face to face about the chances of a negotiated Middle East settlement.

As if to forestall Arab objections, Hassan said, "There is no shame in discussing things with one's enemies."

Jordan appeared anxious today to distance itself from the Moroccan move, special correspondent Samira Kawar reported from Amman. Its information minister, Mohammed Khatib, cautiously said it was "premature" to give a reaction to the meeting although his initial feeling was that it was unlikely to "yield any positive results" toward a comprehensive Middle East peace and "might lead to further fragmentation in inter-Arab relations."

In Tunis, a spokesman for Arafat's PLO said the PLO leadership had been "taken by surprise" by the visit and would issue a statement later.

Marxist Palestinian organizations condemned the meeting, however. George Habash, leader of the Popular Front for the Liberation of Palestine, said Hassan's reception of Peres was "a flagrant violation of the Arab League charter," And the Democratic Front for the Liberation of Palestine said that the talks threatened "the core of the Palestinians' rights and their representation."

In Lebanon, leftist newspapers predicted that Hassan would meet the same fate as Sadat and security was increased at the Moroccan Embassy. Lebanese Prime Minister Rashid Karami predicted that the talks would lead only to more disputes among Arabs, special correspondent Nora Boustany reported from Beirut.

The official Soviet news agency Tass criticized Peres' trip, saying it seemed like a U.S.-inspired attempt to draw Arab countries into separate Middle East peace deals, Reuter reported. But in London, a spokeswoman for the British Foreign Office said, "We wish this imaginative initiative well."

Western diplomats here suggested that both Hassan and Peres were risking a great deal less than then-president Anwar Sadat of Egypt and Menachem Begin, then Israel's prime minister, did in 1977 when they began the first direct Arab-Israeli negotiations, which led to a separate peace treaty two years later. The two leaders met 10 times, the last in June 1981, four months before Sadat was assassinated.

Since Morocco, by geography, is far removed from Israel's physical security concerns, any meeting of the minds reached here would have to be worked out further by Israel and the Arab states directly involved in the Arab-Israeli dispute. Hassan's greatest immediate risk in meeting with an Israeli leader, according to observers, was that he would provoke Libyan leader Moammar Gadhafi into denouncing the treaty of union between Libya and Morocco and once again arming and financing the Polisario insurgents contesting Moroccan sovereignty in the disputed Western Sahara.

Gadhafi today was quoted by Libya's official Jana news agency as expressing disbelief about the meeting and warning that it would "represent a grave violation" of the treaty.

But failing a Libyan decision to resume aid to the Polisario, western diplomats said, Hassan may be counting on Gadhafi's displeasure to remove lingering doubts in the Reagan administration about Morocco's wisdom in signing the largely dormant treaty with Libya in the first place.

If the current talks do lead to serious negotiations, Hassan may hope to increase U.S. aid to Morocco -- running annually at between $125 million and $140 million -- to the much larger amounts enjoyed by Israel and Egypt, according to diplomats.

If the talks fail, the diplomats said, Morocco's willingness to meet Peres can be expected to stand Hassan in good stead with Moroccan Jews -- who make up the largest ethnic group in Israel -- and with Israel's friends in Washington.

Peres runs few potential dangers, according to diplomats who noted that the talks here represent a respite from the unfolding scandal in Israel involving an alleged security services cover-up of the beating deaths of two captured Arab terrorists.

The diplomats also suggested that the talks could prove politically useful at home if Peres decides to provoke a showdown with his Likud coalition partners before October when he is due to relinquish power to them.

Traditionally, Moroccan Jews in Israel have voted for Likud, largely in reaction to their early days as immigrants when they felt they were mistreated by the Labor Party, which ran the country from the creation of the state until 1977.