Israeli Prime Minister Menachem Begin said yesterday that his country is willing to provide air cover for American transport planes and to store U.S. armored vehicles as part of a broadened strategic relationship with the United States.

While Begin sketched a grandiose plan of military cooperation to reporters at a Blair House luncheon, Pentagon officials cautioned in a background briefing that the scheme is still in the conceptual stage and will have to be reassessed if Congress refuses to approve the sale of five airborne warning and control system (AWACS) planes to Saudi Arabia.

A Pentagon official left the clear impression with reporters that it would be against Israel's short- and long-term military interests for it to help opponents block the AWACS deal President Reagan is trying to push through Congress.

As the AWACS battle heats up, the administration is holding out to Israel the prospect of letting it spend $200 million to $300 million in U.S. military aid for equipment made in Israeli factories. Currently, most of this money goes to American factories.

On top of that, defense officials said, the administration has promised to send a team to Israel on Sept. 20 to determine what additional equipment the United States could buy from Israel for its own forces. This delegation will be headed by Gerald D. Sullivan, a Middle East specialist in the Pentagon's research and engineering office.

For the longer term, officials said, defense executives of the United States and Israel will meet frequently over the coming months to explore ways their military forces could jointly combat the Soviet threat in the Middle East.

However, they said Israeli pleas for a written memorandum of understanding on this cooperative effort were unnecessary. "The effort is already launched, so we don't need the pad," said one official. "We don't have to write 20 pages of foolscap about it."

Israeli sources said that while Begin left for New York pleased with the progress of the Pentagon talks, he still wants the agreement spelled out in a memorandum and will press that point in future negotiations.

At his luncheon with reporters, Begin said he had come here with a detailed proposal covering six areas of potential cooperation. Athough he was guarded about describing them in detail, he did say the Israeli plan included these points:

The prepositioning of large amounts of American weaponry, including tanks, in Israel for emergency use by the Rapid Deployment Force being put together by the United States to deal with possible crises in the Mideast and elsewhere.

Israeli willingness to supply an air-cover umbrella extending far into the eastern Mediterranean to protect U.S. transport airlifts if needed.

The use of Israeli ports by U.S. naval vessels for docking, repairs and recreation.

Allowing the U.S. Air Force to use the two air bases being constructed in Israel's Negev region--an offer that includes building an additional runway for American use or extending the existing runways to accommodate large planes.

However, as Secretary of State Alexander M. Haig Jr. made clear Thursday, the United States, concerned about a hostile reaction in the Arab world, wants to limit military cooperation with Israel to much more modest measures for the immediate future.

Among the things the United States has in mind, Pentagon officials said, is a joint U.S.-Israeli air defense for those parts of the Middle East threatened by Soviet forces. Arab nations, if the administration gets its way, would also be knitted into the air defense net, with Saudi Arabia's AWACS aircraft deemed a vital part of it.

Without the Saudi AWACS, said a defense official, the whole air-defense plan would need to be revamped because of the big hole that would be left.

As for accepting Begin's offer to store tanks and other heavy equipment on Israeli soil where U.S. forces could pick it up quickly to fight in the Middle East, Pentagon briefers shied away from embracing the idea publicly. Instead, they stressed the possibility of sending aircraft and armor into Israel temporarily for repairs and routine maintenance.

The strategic arrangements being worked out most likely will include a commitment by Israel to use its ships to help protect the sea lanes in the eastern Mediterranean, defense officials said.

After meetings between Defense Secretary Caspar W. Weinberger and Israeli Defense Minister Ariel Sharon at the Pentagon on Wednesday and yesterday, the two governments issued a communique that said, in part, that "it was concluded that joint Department of Defense and Ministry of Defense groups should meet to discuss strategic problems of common concern and to present recommendations for spheres of cooperation to help deter Soviet aggression against the whole Mideast region."

The joint meetings on strategic arrangements will be held in November and December, according to the Pentagon.