Prime Minister Menachem Begin has termed the month-old Israeli occupation of southern Lebanon a qualified success, and says U.N. forces appear to be serious about controlling the territory they are taking over.

"They appear to mean business," Begin told a closed-door meeting of the Knesset's foreign affairs and Defense Committee Tuesday according to reports of the meeting yesterday. "They have orders to fire at the terrorists if need be."

Begin said that the Israeli military operations, launched March 16 in response to an attack by Palestinian guerrillas that killed 35 Israelis near Tel Avi, had succeeded in pushing the Palestinian guerillas north of the Litani River.

He said the Israeli forces had shattered Palestinian camps and strongholds in southern Lebanon, and added that huge weapons dumps had been captured or destroyed.

Israeli officials have expressed concern, however, that the U.N. troops now moving into the area to replace the Israeli forces will be unable to stop the guerillas from returning to southern Lebanon.

In Beirut Lebanese Interior Minister Salah Salman charged yesterday that Tuesday's announcement that Israeli forces were pulling back from the northeastern section of the zone they had occupied had resulted in only a "microscopic" withdrawal thus far.

"This withdrawal did not conform to the plan we knew about," Salman said.

Diplomatic sources in Beirut suggested that the Israelis had pulled out of seven villaged in southern Lebanon, but that they were retaining strategic positions on surrounding high ground to ensure that there is no return of Palestine guerillas.

In Washington, a State Department spokesman said of the partial Israeli pullout, "We expect the momentum of the Israeli withdrawal to increase in the days ahead."

Begin, in Tuesday's briefing, said Israel could "expect the terrorists to try to come back down south" as Israeli forces continue to withdraw.

He added, however, that "we can only assume that the U.N. intermim force will make it difficult for them."

Meanwhile, the first contingent of troops from Nepal arrived at Tel Aviv's international airport yesterday to join the U.N. force in southern Lebanon. The 30 soldiers were taken to an army transit station to await the arrival of the remainder of the 600-man Nepalese force.

While Israel has scheduled another small pullback in the central sector of southern Lebanon for Friday, officials have said Israel will not withdraw completely until the U.N. interim peace force has deployed all its troops, which eventually are to number 4,000.

U.N. Secretary General Kurt Waldheim will fly to the Middle East this weekend to discuss the situation in southern Lebanon and visit the U.N. force, a spokesman said in New York.