Israeli armored units rolled deep into southern Lebanon yesterday as the government of Prime Minister Menachem Begin discarded its earlier pledge to clear out only a six-mile deep "security belt" of Palestinian guerilla bases in these rocky and battle-scarred hills.

"We are not talking anymore about a security belt," Israeli Chief of Staff Lt. Gen. Mordechai Gur said in an Israeli radio interview. Instead, he said, Israel is seeking a political agreement that would cover all of southern Lebanon and prevent guerrillas from coming back into the border area to launch rocket, artillery and terrorist attacks against Israel.

Early yesterday, Gur said Israeli forces held a line that averaged between six and nine miles deep into southern Lebanon. By late last night, however, Defence Minister Ezer Weizman disclosed on television that three Israeli armored colummns rolled through some 20 miles and occupied all of southern Lebanon to a point just below the coastal city of Tyre and the Litani river.

Weizman said Israeli forces had no intention of moving into Tyre or across the river, where the Syrian Army is based, but will stay in its present positions until it is time to "pack our bags and go home."

Palestinian rocket fire into northern Israel appeared to have subsided. Israel has clamped heavy censorship on reports of fighting from this area, but the Israel military has reported heavy fighting along the border and earlier confirmed that at least two civilians are killed and 12 wounded in artillery attacks in northern Israeli.

The lighting-like Israeli advance, which reportedly met very little resistance, may have been based on the virtual certainty that the United Nations was about to pass a resolution yesterday calling for Israeli withdrawal from southern Lebanon and establishment of a U.N. peacekeeping force to prevent guerrilla activity in the area.

Although no detailed explanation was given to explain the greatly expanded Israeli sweeps, it appears that may have had two other major goals.

To mop up all pockets of Palestinian resistance anywhere in the general border area. Some sources here suggest that the Israeli public war plan intentionally was deceptive all along about staying within a six-mile zone. The real intent, they suggest, may have been to mislead the Palestine guerrilas, to strike throughout all of southern Lebanon by land and air in an attempt to sow confusion and to cause as much damage to the Palestine Liberation Organization as possible in a three- or four-day blitzkrieg.

To push the Palestinians far enough back along a solid line so that their artillery and rockets could no longer reach settlements in northern Israel. The Palestinians have Soviet-built weapons which can reach far beyond six miles, including newly supplied 130mm guns that can fire 18 miles and older 122 mm and 155mm weapons with a range of 10 to 12 miles and Soviet-built Katyusha rockets that also can hit targets 12 miles away.

Although Israeli officials repeatedly stress that they have no intention of staying in southern Lebanon and, as was stated yesterday, no intention of setting up a military government, there is also a widespread feeling here that the Israelis will not be out quickly and there is little confidence that the United Nations can do the police job.

Just hours before the U.N. vote today Gur said that "the U.N., through-out its whole history, could not do such a job on its own unless there was an agreement between the two sides," such as the U.N. role between Egypt and Israel, and between Syria and Israel, which, he said, works well because there are agreements.

U.N. forces operate buffer zones in the Sinai Peninsula between Egypt and Israel and in the Golan Heights between Syria and Israel.

The partners in such an agreement, Gur said, would be Israel and the Lebanese government, which has admitted it cannot control the south, plus the Lebanese Christians, who have "suffered much" at the hands of the Palestinians, and the Syrians, who have the biggest military force in Lebanon but which keeps it above the Litani River.

"If all three elements, and mainly the Syrians, will understand they have to stop terrorism, then it will be stopped," he said. "If the U.N. comes alone, they might have trouble."

Meanwhile, Israeli opposition leader Shimon Peres hit a nerve here for publicly announcing that the army would not withdraw until some arrangement was agreed upon.

peres said this ties the hands of Israel, makes it difficult to get out if the government decides on its own that it wants to, and holds out the prospect of a long war of attrition with both sides shelling each other.

It is precisely that theme that worries many outside observers here, including U.S. and Western diplomats who say the comparison with the U.S. experience in Vietnam is very apparent.

"Now that we're in, how do we get out," is the same problem all over again, says one diplomat.

If the Israelis push the Palestinian artillery back far enough so it hits troops instead of civilians, then protecting the troops instead become the rationale for staying, a cycle that also enveloped the United States in South Vietnam.

Many western diplomats therefore want to see Israel withdraw promptly before the situation becomes a war of attrition from which neither side can withdraw gracefully and before the Jerusalem government becomes even more politically isolated than it is now and before the continues Israeli presence here also isolates Egyptian President Anwar Sadat and crushes hope for peace on that front.

Meanwhile, the signs of filitary sucess for the Israelis were apparent here today.

Just outside this Moslem village, about seven miles inside southern Lebanon, the sporadic return fire of Palestinian batteries from areas to the north was absent yesterday, though there had been some heavy fighting in this region just one day earlier.

Along the road Arab children, mostly Lebanese Christians shout "shalom" in friendly welcome to Israel soldiers.

Yet the sight of young Christian girls in their best white dresses promending on Palm Sunday around demolished tanks and alongside homes pockmarked with two years of shelling by Palestinian opponents to the Christian militia and by the sudden Israel sweep, provide a grim reminder of the toll.

A drive along some 30 miles of the border also revealed no signs of guerilla shellings that had kept a sizable chunk of Israel's northern population in underground shelters this week.

In Maripayoun, about six miles into southern Lebanon, a few local inhabitants - also Christians - who gathered in the unpaved and ramshackle town square, also spoke approvingly of the Israelis.

"The Palestinians shelled us for two years,, said grocer Kamel Razzouck. "So of course things are better now. It's the first time we've had peace in two years."

"For the time being, "Israel is the only country that can solve our problems, added Tarik Tourmal. But it's our country and we want to see the Lebanese Army come and the Israelis go. They said they only came to help."

A few years ago, there were about 10,000 people in this town. Today there are barely 3,000. Many of those who fled are now choking the streets of Beirut in another side of the tragedy not seen from this side of the battle line.