For the fourth time in six days, Israeli warplanes struck deep into Lebanon today, bombing Palestinian positions and destroying five bridges spanning two of Lebanon's major rivers.

Although the air strikes were described as preemptive and not retaliatory, they followed by about two hours a warning by Deputy Defense Minister Mordechai Zippori that Israel intended to "avenge" the deaths of three civilians killed during a Palestinian rocket attack yesterday on Nahariya, a Mediterranean resort town. Seventeen Israelis were wounded in simultaneous rocket attacks yesterday on Kiryat Shemona in Galilee near the Lebanese border.

The Army command said today's extensive raids hit a regional headquarters of the Popular Democratic Front of the Liberation of Palestine near Damour on the Mediterranean coast, an Arab Liberation Front headquarters south of Sidon and "training and terrorist departure" bases south of the port city of Tyre.

In addition, the Army command said, Israeli jets destroyed three bridges across the Zaharani River and two across the Litani. An Army spokesman said the bridges had been used as "traffic arteries for terrorist reinforcements in the south."

The cutting of the bridges between the Mediterranean coast and Palestinian strongholds in south-central Lebanon raised fears among Palestinian and leftist Lebanese groups that Israel is planning to invade Lebanon again, Washington Post correspondent David B. Ottaway reported from Beirut.

[Palestine Liberation Organization leader Yasser Arafat predicted today that Israel would invade Lebanon within two weeks and he charged that Israeli troops were massing at the border in preparation. There was no confirmation of this allegation.]

The army spokesman said the Israeli pilots reported "good hits" on all intended targets, and that all the Israeli planes returned to their bases. aThere were no Israeli reports of casualties but Palestinian and Lebanese sources said in Beirut that at least 28 persons were killed and 30 wounded. In all, they said, 58 persons have been killed by Israeli raids during the past week.

The Army refused to identify the type of planes used in the raids, although the Voice of Palestine radio in Beriut said 12 U.S.-supplied F4 Phantom fighters were used in the attack on Damour.

After today's air raids, the Israeli command said, artillery shells from Palestinian positions in southern Lebanon fell in the western Galilee region of Israel. The Army said Israeli artillery along the Lebanese border returned the fire.

It was unclear tonight whether the current attacks and counterattacks represent a brief spasm of hostilities between Israel and the PLO or are the beginning of a long duel of attrition across the border.

But in an interview on Israeli radio tonight, the Israeli Army chief of staff, Lt. Gen. Rafael Eitan, said the Israeli Air Force had shifted its emphasis in the past four days away from direct military objectives and toward the facilities the PLO is trying to maintain in southern Lebanon.

"In the past, we attacked only direct military objectives and not the infrastructure," Eitan said. "Now, these attacks will make it ever more difficult to assemble a military force. This is not a final solution. Their ability is being cut down. If this continues, we shall have to think of other ways of attacking them."

The sternest warning against Palestinian guerrillas came from Zippori at the funeral in Nahariya for two Israeli victims of last night's shelling.

"The government of Israel is responsible for the defense of the country and the security of the people, and we shall do our best not only to prevent [future attacks] but to punish those conscienceless murderers that have done this shameless thing," Zippori said.

Zippori said the PLO has accumulated "quantities of sophisticated conventional arms," including tanks, heavy artillery and launchers capable of firing 41 rockets from one vehicle.

"We knew they were trying to prepare themselves to hit us and we are trying to prevent that. And I think we prevented many, many operations that would have been carried out if we didn't strike them. But it seems that the lesson wasn't enough and we shall have to deal with them in the future," Zippori said.