Israeli Prime Minister Menachem Begin told Secretary of State George P. Shultz today that Syria is staging a massive military buildup aided by a $1.5 billion infusion of sophisticated Soviet equipment and 4,000 Soviet military advisers.

The Israeli charges were made when Shultz met with Begin on the first leg of a shuttle mission between Jerusalem and Beirut. Shultz is seeking to conclude successfully the four-month negotiations on an agreement for the withdrawal of Israeli forces from Lebanon.

U.S. officials accompanying Shultz said he initially reacted to the Israeli charges--which were given in a detailed briefing by Israel's chief of military intelligence--by saying that Syrian activities were "a source of concern."

In private, the officials said the United States had not yet had time to assess the Israeli data and check it against U.S. intelligence reports.

As a result, they added, it was not immediately clear whether Begin's presentation was an indication of genuine Israeli concern about a threat from Syria or a diversionary tactic aimed at winning U.S. sympathy at the outset of anticipated hard bargaining over the Lebanon withdrawal question.

Some details of the presentation by Maj. Gen. Ehud Barak had been leaked to the Israeli press prior to Shultz's arrival today. Some sources said Begin's decision to stress the matter so strongly in his meeting with Shultz was an attempt to use the heavy press attention being accorded Shultz's Middle East trip as a means of getting the Israeli concerns publicized widely in the United States.

The Israelis charged that the Soviets have helped Syrian President Hafez Assad to rebuild his forces through massive replacement of tanks and jet aircraft destroyed by the Israelis during the fighting in Lebanon last summer. In addition, the Israelis contended, Soviet advisers are operating advanced, SA5 ground-to-air missile batteries within Syria and are operating sophisticated electronic equipment for the Syrians.

Until now, the official U.S. position toward the alleged Syrian buildup has been that the United States has heard reports of extensive troop movements within Syria but has been unable to confirm them.

Egyptian President Hosni Mubarak, who conferred with Shultz in Cairo yesterday, is reported to have told the secretary that a military buildup is under way in Syria but that it is aimed at putting pressure on Israel to withdraw from Lebanon.

Mubarak reportedly told Shultz that Assad has no intention of starting a new armed conflict with Israel and will not allow himself to be pressured by the Soviets into an actual confrontation.

The Syrians have announced that they would welcome a visit from Shultz during his Middle East mission. While flying here from Cairo, the secretary said he hopes and expects to have a meeting with Assad since the ultimate aim of U.S. policy is the withdrawal of all foreign forces, including those of Syria and the Palestine Liberation Organization, from Lebanon.

The Israeli charges about Syria, which received heavy radio and television coverage tonight, tended to overshadow the main purpose of Shultz's visit, which is to press for a conclusion of the Lebanon troop withdrawal negotiations. The immediate indications were that several days of discussion and intensive mediation by Shultz will be required before it is clear whether that goal is within reach.