The White House, concerned by an upsurge of rumors that Israel is preparing to attack Palestinian forces in southern Lebanon, yesterday called on "all those involved to show the utmost restraint" in avoiding a major new Middle East crisis.

Deputy White House press secretary Larry Speakes, speaking in Barbados where President Reagan was vacationing, said there are no immediate plans to send the president's special Mideast envoy, Philip C. Habib, back to the region. But Speakes said Habib, who last summer negotiated the fragile cease-fire between Israel and the Palestine Liberation Organization, "stands ready" to go if the need arises.

The U.S. appeal for "maximum caution" came as Lebanese President Elias Sarkis charged that Israel was massing two armed divisions along the border between the two countries.

Although U.S. officials publicly refused to confirm or deny the reports, some said privately that Washington has no evidence of a large-scale, new Israeli buildup of forces in the border area. These sources said that, despite intensive consultations with the Israelis, the United States does not know what Prime Minister Menachem Begin's government intends to do.

"They're not telling us, and we frankly don't know what their intentions are," said one official, who declined to be identified. "At this point, any American official who claims to know is simply talking off the top of his head."

But the officials also acknowledged, as the White House statement said, that "this is a time for maximum caution." They expressed particular concern about the possibility of having to confront a new flare-up of Mideast violence at a time when the attention of Secretary of State Alexander M. Haig Jr. and other senior administration officials is focused on trying to defuse an impending clash between Britain and Argentina over the Falkland Islands.

In fact, the danger posed by the Falklands crisis was understood to be one of the arguments being used by the United States in urging restraint on Begin. There are strong parallels between the current tensions and the situation last December when Israel, acting in the aftermath of the military crackdown in Poland, stunned Washington by annexing the occupied Golan Heights.

That caused the Reagan administration to retaliate by suspending an agreement for greater U.S.-Israeli strategic cooperation. While that move provoked Begin into an unprecedented public temper tantrum, the incident is generally thought to have made him more wary of seizing on sensitive periods of world tension to do things that antagonize the United States.

Similarly, the administration has made known that it plans to follow Israel's scheduled April 25 return of the Sinai Peninsula to Egypt with a new drive to work out an autonomy agreement for the Palestinian inhabitants of the Israeli-occupied West Bank and Gaza Strip.

An invasion of Lebanon at this time almost certainly would derail that effort and force the United States to consider seriously abandoning the Camp David peace process, which is staunchly supported by Israel, and seeking other policy approaches to the Middle East, which would be much less to Israel's liking.

But, while the United States was known to be underscoring these arguments to the Israelis, administration officials also acknowledged that if the Begin government does intend to strike at the PLO strongholds in southern Lebanon, there are several factors that might well persuade Begin that this is the best and most logical time.

Chief among them is the pressure to move at a time when Egyptian President Hosni Mubarak, whose main priority is to regain the Sinai, would be constrained from reacting strongly to a strike against Lebanon. If Israel were to act later, after the Sinai is back in Egyptian hands, Mubarak's desire for a rapprochement with the Arab world could force him to choose sides in a way that would scuttle the Egyptian-Israeli peace accord.

In addition, the internal dissension besetting Syria and Iraq's paralysis as the result of its losing war against Iran have heightened the division and drift within the Arab world to the point where it would be unable to patch together any coordinated defense of Lebanon and the PLO. The continuing world oil glut also would prevent Saudi Arabia and the other oil-producing Arab states from staging another boycott of the kind they used so effectively in 1973.

Awareness of these factors is believed to have played a big part in the increasing bellicosity that Israel has exhibited toward the PLO in recent days. Using as its rationale the murder of an Israeli diplomat in Paris and the violent protests of West Bank Palestinians, the Begin govenment has been very outspoken in charging that these provocations can be traced back to the PLO in Lebanon and warning that it might have to attack them at the source.

U.S. officials are known to be especially concerned that a recent article by the Israeli governor of the occupied territories, Menachem Milson, outlining the need to eliminate PLO influence and intimidation in the West Bank, was intended as a signal that this goal cannot be achieved without going after the PLO inside Lebanon.

These officials are also worried that the ominous-sounding Israeli statements made after the diplomat's murder clearly deviate from U.S.-Israeli understandings that violations of the cease-fire should be defined as provocative actions across the Israeli-Lebanese border. During the past week, the State Department has taken pains to make clear that the United States still holds this view and expects Israel to do the same.

In the U.S. view, the main pressure for action against Lebanon within the Israeli government comes from Defense Minister Ariel Sharon, who is regarded as mistrustful of U.S. intentions and far less concerned than Begin about incurring Washington's displeasure.

Several weeks ago, Begin was deterred at the last minute from a planned move against Lebanon as the result of stern warnings that it would have had severe consequences for U.S.-Israeli relations. But the pressure on him from Sharon and other hawks has continued, and U.S. sources conceded yesterday that it is still an open question whether their impatience for action or fear of serious new strains in Israel's ties with Reagan will be decisive in determining which course Begin takes.