The State Department said yesterday the U.S. ambassador to Israel spoke with "complete accuracy" when he charged that former Israeli defense minister Ariel Sharon had outlined his plans to invade Lebanon six months before Israeli troops began their march to Beirut.

The department pointedly disputed Sharon's denial of Ambassador Samuel W. Lewis' account of a December 1981 conversation with Sharon and President Reagan's special Mideast envoy, Philip C. Habib.

In an Israeli television interview Wednesday, Lewis said that Sharon had outlined his invasion plans "in some hypothetical detail" to Habib in the conversation six months before the invasion. Sharon subsequently denied that he had told U.S. officials "details of any operational plan" in Lebanon.

The State Department initially refused comment on the dispute. Spokesman Edward Djerejian said today, "We can confirm that Ambassador Lewis has described the U.S. position in this matter with complete accuracy. We strongly object to any suggestions to the contrary."

By challenging Sharon's version of the dispute with Lewis, the administration brought into the open what has long been known: that the United States, Israel's principal ally and supporter, regards Sharon as untrustworthy and has grave misgivings about the consequences for Middle East peace if he were to become Israel's leader.

What effect that might have on Sharon's political future or on U.S. relations with Israel's coalition government was not clear. Sharon, currently minister of trade and industry, is a powerful force in the unity government's conservative Likud bloc, and his supporters are likely to charge the United States with interfering in Israel's internal affairs.

However, the U.S. position could strengthen the hand of Prime Minister Shimon Peres's Labor Party, which opposes Sharon's hard-line advocacy of military solutions rather than negotiation with Arab adversaries. The Labor Party has sought to create the impression among Israeli voters that Israel cannot afford policies or leaders that would threaten its ties to the United States.

The dispute also raised questions here about how much the United States knew in advance about the Israeli planning of the June 1982 invasion and whether the administration could have done more to prevent the bloodshed and political chaos.

A senior U.S. official, who spoke with reporters yesterday on condition he not be identified, said that Sharon, in his December 1981 meeting with Habib, "was giving us his personal concepts."

The official stressed that the "meeting was not a review of a specific battle plan," and he added that while it was clear that Israel was considering military action, the United States was not informed about dates, objectives and strategies.

He recalled that in the six months before the invasion, "there were numerous public indications that military action by Israel was likely. We made repeated efforts at all levels to discourage any actions which would lead to war."

"We want to emphasize that beyond what was generally and publicly known, the U.S. government had no prior knowledge of the invasion of Lebanon," the official said. "We had no prior knowledge that Israel had decided to go to war . . . .

"During this period, we had frequent contact with the Israeli government then headed by Prime Minister Menachem Begin . We were seeking guarantees from the Israelis that they would not invade under various scenarios," the official said.

He added that the "personal concepts" described by Sharon to Habib "fell into that category" and had prompted an "official, vehement and very negative response" from Habib. "It was followed by a period of intense diplomatic activity at all levels in which we were saying to the Israelis, 'Don't do it.' "

In his book, "Caveat," Alexander M. Haig Jr., who was secretary of state at the time, described U.S. diplomatic efforts to dissuade Israel from invading. In particular, Haig wrote, that after a February 1982 briefing from Israel's military intelligence chief, he instructed Lewis to warn Begin of "far-reaching consequences for our relationship."

Haig also said that in late May, Sharon, on a visit here, "shocked a roomful of State Department bureaucrats by sketching out two possible military campaigns: one that would pacify southern Lebanon and a second that would rewrite the political map of Beirut in favor of the [then Israeli-allied] Christian Phalange."