Prime Minister Menachem Begin asserted yesterday that a senior Reagan administration official, who warned that strengthened U.S.-Israeli security ties could be affected if Congress blocks the sale of sophisticated radar planes to Saudi Arabia, "didn't know exactly what he's talking about."

"It won't affect it all, either way," Begin said in reference to whether there is a link between the Saudi plane sale and the agreement he reached with President Reagan last week to broaden military cooperation between the two countries.

At a Pentagon background briefing Friday, a senior Defense Department official cautioned that the scheme is still in the conceptual stage and will have to be reassessed if Congress refuses to approve the $8.5 billion Saudi deal which includes five Airborne Warning and Control System (AWACS) planes.

Under the rules of the briefing, the official cannot be identified.

Begin, asked about that warning during an interview yesterday on "Meet the Press" (NBC, WRC), said he had seen the statement "to the effect that if that deal doesn't go through, then there won't be any strategic cooperation," and he added: "I believe that official didn't know exactly what he's talking about."

"Anyhow, I can tell you that I heard the president," Begin said. "I heard the secretary of state, the secretary of defense. I never even heard such an allusion about such a possibility, and I don't think they would have hidden such a possibility from the Israeli delegation."

During his official visit here last week, Begin stressed repeatedly that Israel's proposals for greater strategic cooperation were related exclusively to guarding against the threat of Soviet penetration in the Middle East and should not be regarded as compensation for the Saudi sale or an attempt to obtain enhanced U.S. protection in its continuing conflict with the Arab world.

He took the same line yesterday, emphasizing that he regards the sale of AWACS and enhancements for Saudi jet fighters as "a danger to Israel's security."

He added: "But the strategic cooperation between the United States and Israel is of the highest importance to both countries and to the free world. I think maybe it will go on. It should go on."

The Pentagon official, whose briefing prompted Begin's remarks, was contacted by The Washington Post yesterday but refused to make any further comment on the grounds that he had not seen the prime minister's statement.

However, the incident appeared to point to continuing differences within the administration about how to deal with Israel's opposition to the Saudi sale and its impact on pro-Israeli members of Congress when they vote next month on resolutions to veto the arrangement.

Before Begin's visit, some administration officials had talked about warning Israel that the price for improved relations would be a softpedaling of its hostility to the Saudi sale, and there are forces, particularly in the Pentagon, who advocate maintaining that tough line.

However, their position appears to be at variance with the attitude expressed by Secretary of State Alexander M. Haig Jr. at a Thursday news conference.

He expressed open appreciation for the low-key stance adopted by Begin toward the AWACS dispute during his visit, and the secretary also seemed to agree with Begin's view that the outcome of the AWACS fight should not be related to increased military cooperation.

That idea ties in with Haig's desire to combat Soviet influence in the Mideast by weaving together a "strategic consensus" of the region's pro-western countries.

In talking with The Post yesterday, the Pentagon official did clarify another point of apparent difference with Begin. The Israelis want the cooperation agreement spelled out in a memorandum of understanding, and Haig endorsed the idea as a "launching pad" for the proposed joint venture.

At the Friday briefing though, the official seemed to dismiss the idea with the remark: "The effort is already launched, so we don't need the pad. We don't have to write 20 pages of foolscap about it."

But Begin insisted yesterday that he still wants a written agreement and said he hoped it could be signed when further high-level talks are held next month and in November.

In response, the Pentagon official said yesterday that his remarks Friday referred only to the situation as it stood then, and added that "as progress is made on the specifics, there may be good sense to signing a memorandum in November."