Israeli Prime Minister Shimon Peres will meet with President Reagan here Tuesday, but officials on both sides said yesterday that the talks are unlikely to change U.S. unwillingness to help mediate a new agreement for withdrawal of Israeli forces from southern Lebanon.

They also said the meeting is not expected to produce any immediate agreement on additional long-range U.S. economic aid to help Israel overcome its financial crisis.

Instead, the officials added, the emphasis will be on explaining in detail to the administration how Peres' national unity government plans to shore up Israel's deteriorating economy and examining how new U.S. aid would be most effective when the Israelis put their recovery program into practice.

U.S. reluctance to take on a new mediating role in Lebanon, where its earlier policy failures led to heavy loss of American lives and prestige, stems from Secretary of State George P. Shultz's talks in New York this week with Israeli Foreign Minister Yitzhak Shamir, Lebanese Prime Minister Rashid Karami and Syrian Foreign Minister Farouk Charaa, as well as earlier soundings in the Mideast by Richard W. Murphy, assistant secretary of state for Mideast affairs.

These explorations have convinced the United States that the two sides remain too far apart for a new initiative to be successful. A senior administration official, who spoke with reporters at the White House yesterday on condition that he not be identified, said:

"The United States is not in any formal mediation or negotiation . . . . We have been exploring whether there is a helpful role to play. We're not sure there is one, based on our contacts so far."

Other U.S. officials said the principal sticking point involves Israel's insistence that Gen Antoine Lahad's Israeli-backed southern Lebanon force keep an important role in ensuring that an Israeli pullout from southern Lebanon is not followed by the return of Palestine Liberation Organization guerrillas.

Lebanon and Syria, which has a veto over the Karami government's actions, insist that Lahad's force be disbanded. They contend that Israel's northern border can be safeguarded against terrorist attacks by the Lebanese army and the U.N. forces in southern Lebanon.

"The United States can't tell Israel it should give up Lahad's force, because the Lebanese army doesn't work," one U.S. official said privately. "The Israelis would just laugh at us, and rightly so. But we're also telling the Israelis that Lebanon and Syria won't accept continuation of the Lahad force.

"So the situation is deadlocked for now, and it will stay that way until one side or the other flinches," he continued. "At the moment, there's no sign of which that will be or when."

Michael H. Armacost, undersecretary of state for political affairs, will go to Israel later this month to continue talks started last year by his predecessor, Lawrence S. Eagleburger, and David Kimche, director general of the Israeli foreign ministry.

Officially these talks are to maintain a dialogue on U.S. and Israeli activities in the Third World; however, they have provided a framework for other initiatives. U.S. officials maintained yesterday that Armacost's trip is not prompted by the Lebanon situation, and they said that if the United States does undertake further mediation efforts, they expect Murphy to play the main role.

However, they acknowledged that Armacost expects to visit other Mideast countries. They also said that his trip, which will come after the contesting parties have had a chance to reflect on U.S. conditions for getting involved, might provide new opportunities for progress.

In the economic area, aides to Peres said earlier this week that he was not coming to Washington for "a beggar's handout" and did not contemplate immediately seeking an increase in the $2.6 billion in economic and military aid that Israel will receive. Earlier there had been speculation that Peres would request as much as $1 billion in supplemental help.

However, the senior U.S. official said in his briefing that "there has been no discussion yet of actual figures" between the two governments. He added that the United States has only partial details of the recovery plan Israel is putting together.

"There are lots of possibilities," he said. "But it would be premature to talk about what kinds of things might be done. We have to get the Israeli program fully spelled out first."

Israeli sources said some forces in the Peres cabinet believe that the prime minister should make specific requests and get a U.S. aid pledge nailed down quickly. However, they added, Peres' preference is to seek a more general commitment that U.S. loans and grants will be available early next year when the shape of his recovery program is clearer and he has a better idea of how he wants to use outside aid.