Secretary of State Alexander M. Haig Jr. yesterday said the United States has reports that Palestinian forces in southern Lebanon are receiving Soviet rockets and artillery, a development he said could jeopardize U.S. efforts to prevent new fighting in that troubled region.

Haig disclosed this information at a news conference called to discuss the results of Egyptian President Hosni Mubarak's official visit, which ended yesterday.

He declined to give details about the type and quantity of the weapons allegedly being received by Palestinian units. But he made clear that "the provision of such armaments" is viewed seriously by the United States as a potential threat to the uneasy truce involving the Palestine Liberation Organization, Lebanese Christian forces, Israel and Syria.

The United States has been concerned in recent weeks that new provocations by the PLO could induce Israel to invade southern Lebanon, which could lead to war between Israel and Syria.

Although Haig did not cite this concern specifically, he did say the situation will require an intensification of American efforts to strengthen the cease-fire arrangements between the two countries. On the main subject of his news conference, Mubarak's visit, Haig was more upbeat.

He said the Egyptian leader's two days of talks with President Reagan left Reagan "with a deep sense of confidence and optimism that U.S.-Egyptian relations will proceed on a sound and profitable basis."

Haig's strong praise for Mubarak appeared to be aimed at countering concern that the Egyptian leader, who became president after the murder of Anwar Sadat last October, might move away from his country's strong ties to the United States after Israel returns the last part of the occupied Sinai Peninsula to Egypt at the end of April.

Mubarak helped fuel that concern when, during his Wednesday White House visit, he failed to mention the Camp David accords, the cornerstone of U.S. Middle Eastern policy. Instead, he emphasized "a national entity" for the Palestinian inhabitants of Israeli-occupied territories.

Since then, however, Mubarak has gone out of his way in public appearances here to stress Egypt's continued adherence to the Camp David peace process and his desire to maintain close ties with the United States.

He repeatedly underscored these themes in a speech at the National Press Club yesterday.

In the address, he pledged to build "new bridges of understanding and friendship" with Israel after the Sinai withdrawal is completed, said that "Camp David remains the most valid mechanism for a comprehensive settlement of the Mideast problem" and described Egyptian friendship for the United States and the West as "one of the pillars" of Egyptian policy.

He also elaborated for the first time on the term "a Palestinian entity." Addressing concerns that he might have meant recognition of the PLO or the creation of an independent Palestinian state, he said:

"We have in mind recognition of rights and not of institutions or organizations. Each side must recognize the other's right to live in peace and security."

But Mubarak also reiterated Egypt's contention that any formula for autonomy in the West Bank and Gaza strip territories must be acceptable to Palestinian moderates, and he again made clear that he is in no rush to push through an autonomy accord. "To rush to any formula for the sake of reaching an agreement would be counterproductive in the long run," he said.

Haig went out of his way to underscore the administration's satisfaction with the visit and to praise Mubarak as a close and valued friend of this country.

He also made clear that the United States plans to express its appreciation in concrete terms by meeting some of Mubarak's requests for an improved military and economic aid relationship.

Haig said that, as part of a program to modernize the armed forces of friendly Middle Eastern countries, the United States will ask Congress to increase military aid to Egypt in fiscal 1983.

He declined to give specifics, but he hinted that the tentative plan to increase military assistance from $900 million to $1.3 billion will be "improved," probably by increasing the amount of loans that Egypt does not have to repay.

Mubarak also said that the administration agreed to his request to give him greater flexibility in using approximately $1 billion in economic aid by earmarking it for "sectorial assistance" rather than for specific projects.

In addition, U.S. and Egyptian officials signed an agreement yesterday raising U.S. Food for Peace financing of Egyptian purchases of American grains from $200 million to $275 million.