The rightist offensive that has brought a new surge of fighting to southern Lebanon was launched by Israel to scuttle the all-but-completed U.S.-backed agreement to thin out Palestinian guerrilla forces in the area, according to informed sources here.

Israeli Prime Minister Menahem Begin's declaration yesterday, four days after the offensive began, that Israel was "ready to discuss a general cease-fire in southern Lebanon" is seen by these sources a moves to shatter the agreement that was near completion.

"Peace was awful close but the Israelis couldn't live with it," said a source who closely followed the lengthy negotations, which now are considered at an end.

The negotiations, in which the United States played a key role, were aimed at securing Israeli approval of the July 25 agreement reached at Chtaura, Lebanon, that would remove all but a token number of Palestinian commandos from the sensitive Israeli border region and replace them with Lebanese army troops.

There is a strong feeling among observes here that the United States, while aware of the Israeli activity, is not interested in pressing Israel at the moment for fear that this would worsen an already sensitive situation.

In Washington, Secretary of State Cyrus Vance said today: "I think it's important there be a cease-fire immediately. We will do what we can to work with the parties involved to keep the situation form getting out of hand."

Begin's offer to discuss a general cease-fire was seen here as a tacit asmission of Israeli involvement in the fighting.

Obervers here are convinced that Israel would rather have the southern Lebanese situation left unsolved - and thus subject to its whims - than achieve its long-held objective of forcing the guerrillas out of the border area.

Begin's offer was also seen as meaningless since Israel refuses to deal with the Palestine Liberation Organization and Lebanon for 30 years has gone through diplomatic contortions rather than deal directly with the Jewish state and thereby be considered a confrontation state.

"The Israelis are changing the rules and demanding a new ballgame," a well-informed source said. "They extracted maximum consessions from the Lebanese and the PLO through the United States and now want to go beyond what the Lebanese and PLO can live with.

"In return for stopping shelling a country they're not a war with, the Israelis were about to achieve their supposed heart's desire, of getting the Palestinians away from their border," the source added.

Under the agreement of July 25, Lebanese army units were in replace Palestinian guerrillas in the border area. The Syrian occupation army has avoided the area to comply with Israeli demands.

Only 250 guerrillas were to be allowed in an area stretching from the Mediterranean sea to a line parallel to the northernmost point of Israel. They were to remain at least six miles from the Israeli border.

The Israeli offer to begin new negotiations with Lebanon, sources here say, was apparently designed to drive a wedge between the Lebanese and the Palestinians, who made accepted major concessions in the chtaura agreement.

Israel now apparently wants the Palestinians out of a much greater area in southern Lebanon than the guerrillas agreed to in the 1969 Cairo agreement, which served as the basis for that in chtaura. Going beyond those terms would be political suicide for PLO leader Yasser Arafat, according to diplomatic observers.

Other Israeli motives for launching the southern Lebanese offensive may have included tactical and political considerations. The Palestinians, who had thinned out their southern Lebanese forces in what they presented as a good-will gesture, were caught off balance. The Israelis may have hoped that the guerrillas would rush reinforcements to the south and walk into an Israeli meat-grinder.