The crisis in Lebanon took a new twist yesterday with the reported involvement of Libyan military forces, but U.S. officials expressed confidence that it will not impair Mideast emmissary Phillip C. Habib's efforts to stave off a large-scale Arab0Israeli conflict.

Habib, who arrived at Dulles Airport yesterday afternoon and made no public comment on his three-week mediation mission in the Middle East, is scheduled to report to President Reagan this morning. An official said Habib is expected to return to the Middle East next week to resume his mission.

The new Israeli air raid into southern Lebanon, reportedly against Libyan SA9 antiaircraft missile positions, brought subdued reactions from the Reagan administration. State Department spokesman Dean Fischer said the United States does not believe that the Israeli strike is "necessarily related to our efforts to defuse the tension brought about by the crisis" in Lebanon."

At the State Department, there was little surprise about the Israeli actin. A senior assistant to Secretary of State Alexander M. Haig Jr. had spoken to reporters only Wednesday of "a Libyan potential for troublemaking" in the already complex Lebanese tangle.

According to official sources, U.S. intelligence has received reports of notably increased shipments of Libyan military equipment into Lebanon since long-term problems there flared into a new crisis last month.

The intelligence reports also indicated that a limited number of Libyan instructors or antiaircraft-operations personnel have entered Lebanon in this period, but the consensus here is that no large contingents of Libyan troops were involved, the sources said.

Israeli radio and television reported last Saturday that 54 trucks carrying artillery and ammunition supplied by Libya had crossed into Lebanon from Syria two days earlier. The weaponry reportedly was destined for Palestinian groups in southern Lebanon.

On Sunday, Libyan leader Muammar Qaddafi and Palestine Liberation Organization chairman Yasser Arafat meet in Libya and issued a statement calling on all Arab states "to adopt a practical stand -- materially, politically and militarily -- on the side of the Palestinian revolution and Syria" in the face of threats and attacks from Israel.

Relations between Qaddafi and Arafat have been strained recently, especially because of Libyan aid to extremist PLO factions that Arafat considers to threat to his position, according to U.S. sources.

In this view, Qaddafi is using the Lebanese crisis to justify increased and more open Libyan aid to such Palestinian groups, on grounds of assisting in their defense against Israel.

Some Beirut reports, which U.S. officials consider unproven but believable, indentified Libya as the instigator of rocket attacks last Sunday night against U.S., Egyptian and Sudanese embassies there.

The Qaddafi-Arafat statement, made public earlier that day, concentrated much of its verbal attack on Sudan for its reconciliation with Egyptian President Anwar Sadat, who is considered an archenemy by Libya and the PLO.

Sadat's meeting next Thursday in the Sinai desert with Israel Prime Minister Menachem Begin, which was announced yesterday, is a particularly striking display of the Egyptian-Israeli solidarity bitterly opposed by Libya and the PLO. The timing of the meeting suggests it is designed to assist Begin in his battle for reelection in the Israeli national balloting June 30.

Regarding a settlement in Lebanon, Israeli radio quoted Habib as telling Begin that Saudi Arabia has been unable to convince the Syrians to withdraw their antiaircraft missile from the Bekaa valley.

Diplomatic source close to the situation said, however, that Syria has displayed willingness for several weeks to withdraw its missiles in time, if this can be done in a way that does not suggest capitulation under pressure to Israel and the United States.

Habib's descussions with Israel and Saudi Arabia's contacts with Syria are two facets of the same drive to defuse the Lebanese crisis, according to the sources.

The Saudis, who reportedly have been exploring a long-term basic solution to instability in Lebanon, are still concentrating their principal effort on a solution to the immediate flashpoint in the Bekaa valley, the sources said.