Israel's new ambassador to the United States, Meir Rosenne, yesterday heatedly denied that his country invaded Lebanon in a search for water resources and expressed the hope that international pressure will force Syria to end its "illegal" occupation.

In his first public address since arriving Sunday to take his post here, Rosenne told the Anti-Defamation League of B'nai B'rith that "a campaign" against Israel seeks to prove that its entire operation in Lebanon "was destined to provide Israel with water resources which is absolutely not the case."

Rosenne said the purpose of the Israeli campaign in Lebanon was "to put an end to the terrorist attacks against Israel."

The Israeli intervention, he went on to say, saved the Christians of Lebanon from being "massacred" because nobody else intervened to save their lives.

Aides said that in discussing the water rights issue, which he twice described as a campaign being waged in the press, Rosenne referred to a Washington Post report Wednesday that Syria has demanded ironclad guarantees against Israeli control of the Lebanese headwaters of an important Syrian river, the Orontes.

The article said many experts believe that the control of dwindling water resources in the area is a key to the U.S.-sponsored troop withdrawal from Lebanon and to the prospects for peace. The article did not state nor imply that water was the goal of Israel's 1982 invasion of Lebanon.

Rosenne, who spoke without a text in a rapid-fire, passionate style, charged that a "double standard" against Israel is being promulgated "as a new international rule and new international law, and Israel is not ready to accept it."

He complained that demands had been made on Israel to withdraw from Lebanon after its invasion a year ago, but that now Syria refuses to withdraw and "nobody reacts."

"We do hope enough pressure will be put on Syria to make it leave the country which it occupies illegally," Rosenne said.

At the State Department, meanwhile, a series of meetings of senior U.S. diplomats from the Middle East pondered ways to advance Syrian withdrawal from Lebanon and thus bring about the pullout of other foreign forces, including those of Israel.

State Department officials said no conclusions or decisions were sought or reached at the gatherings of the diplomats, whose ideas are to be presented to Secretary of State George P. Shultz after his return here from Paris late today.

A senior official dealing with the area said it is obvious that the United States will have to reassert itself on the ground in the Middle East in the coming period. Among the possibilities that have been discussed are a new trip by Shultz, who is expected to be in nearby South Asia in early July, or a trip to the area by Deputy Secretary of State Kenneth W. Dam.

State Department sources said it remains unclear to the United States whether Syrian leaders have staked out a tough position in order to win major concessions in return for a withdrawal from Lebanon, or whether the Syrians are firmly determined not to withdraw in any case.

Among those participating in yesterday's meetings were Nicholas A. Veliotes, assistant secretary of state for Near Eastern and South Asian affairs; special presidential Mideast negotiators Philip C. Habib and Morris Draper; Ambassador to Israel Samuel W. Lewis; Ambassador to Jordan Richard N. Viets; Ambassador to Lebanon Robert S. Dillon, and Ambassador to Saudi Arabia Richard W. Murphy. All are career Foreign Service officers.