Israeli officials began turning their attention to a second phase of their action in Lebanon today, warning that they will consider any infiltration of Palestinian guerrillas now in Beirut into Syrian-controlled territory in eastern Lebanon a violation of the Beirut evacuation agreement.

The officials stressed that Israel will hold Syria responsible for any attacks by the Palestine Liberation Organization from Syrian-controlled territory, but they refused to say what action Israel would take in such a case.

According to the officials, Israel still has not received the final written evacuation plan crafted by U.S. special Middle East envoy Philip C. Habib, nor has it received a formal response to its demand for the return of a pilot shot down in June and the bodies of nine soldiers -- four missing since the 1978 Israeli invasion of Lebanon and five from the current one -- before the evacuation begins.

The officials said they assumed, however, that only "technical issues" were delaying completion of the plan and that they hoped the Palestinian guerrillas would begin leaving Beirut "in the next few days."

In a radio interview tonight, Israeli Defense Minister Ariel Sharon, who met today with Habib in Beirut, said progress was made on a number of issues, including the evacuation of Syrian troops that are also trapped in West Beirut and the return of the pilot and the nine bodies. He said the negotiations apparently were reaching their "final phase."

From the beginning of the negotiations over the fate of the trapped PLO fighters, Israel has divided the process into two phases, the first dealing with the evacuation of the guerrillas from the city of Beirut and Lebanon, and the second dealing with the withdrawal of "all foreign forces" from Lebanon.

The comments of Israeli officials today underscored that this so-called second stage, while less dramatic than the siege of the Lebanese capital and the frantic diplomatic efforts to avert an all-out Israeli assault on the city, is likely to involve protracted negotiations that could lead to a prolonged Israeli occupation of southern Lebanon.

Syrian troops remain in the Bekaa Valley in eastern Lebanon, which Syria historically has considered vital to its own security because of its proximity and easy access to Damascus. There are also PLO forces still in northern Lebanon, concentrated around the port city of Tripoli.

Privately, Israeli officials have hinted at their willingness to acquiesce in the presence of the Syrians in a portion of the Bekaa Valley. But the public posture they will bring to negotiations on this issue is that the Israeli Army will remain in Lebanon until all other "foreign forces" also leave the country.

The officials admit that Israel has no guarantee that the guerrillas, once they are removed from Beirut, will not reenter Lebanon through the Syrian lines in the Bekaa Valley. But they said such a move by the PLO would violate the agreement that Habib has produced over weeks of negotiations.

"We have demanded all along that the PLO leave West Beirut and Lebanon," one official said. "If they return to Lebanon, obviously this is going back on the understanding we are achieving. It is our understanding, and the understanding of all the parties involved, that the PLO will leave West Beirut and Lebanon and will not return. We don't look on this as something temporary."