Lt. Gen. Mordechai Gur, Israel's chief of staff, said yesterday that he hopes the U.N. peacekeeping force will move into southern Lebanon "as quickly as possible to his troops can withdrawn "very fast."

Asked how long his invasion force would remain now that the cease-fire declared Tuesday appeared generally to be holding, Gur said "that depends on the United Nations forces," which began arriving yesterday.

"This I can almost assure you," the stocky general told journalists visiting this hilltop town four miles from the port of Tyre, "if it depends on us, very fast. If it depends on the U.N. it might mean be days."

Gur confirmed reports that Israel decided to expand its original six-mile-deep "security belt" last Saturday because of the accelerated U. N. Security Council negotiations on establishing the buffer force.

"We wanted to wait to discuss the problem with the Americans right there in Washington" early this week, he said. "Our main goal was to create the best infrastructure for the U. N. to deploy in."

When a reporter suggested that meant Israel wanted to "take as much land as possible," he said, "Yes, south of the Litani."

Repeating his desire to pull out quickly, Gur declared:

"I can't say if it will take hours, days or weeks." But he insisted that he understood that Lt. Gen. Gen. Ensio Silvasuo, the U. N. Middle East Force commander, "wants to go fast." The authorized strength of the U. N. force is 4,000 men.

Despite reports of occasional cease-fire violations involving firing initiated by both Israelis and Palestinians, Gur said, "No one has shot from our side" since Israel announced the unilateral cease-fire effective at 6 p.m. Tuesday.

A few rounds of what appeared to be Palestinian-iniated firing reverberated around this largely abandoned town, whose remaining inhabitants had hoisted half to dozen white flags as a sign of surrender.

Gur, standing inchilly, overcast weather with shirtsleeve rolled up and his beret slipped under an epaulet, discounted reports that Israel would like Syrian troops to move into a 200-sqyare-mile are separated by the Litani River from the Israeli-occupied zone to the south.

That rectangle of territory, now the last area where the Palestinians enjoy unfettered military freedom of movement, has been off limits to Syrian troops at Israeli insistence since Syria sent a peacekeeping force into Lebanon in 1976.

"For the time being I see no need for the Syrians to go down (to the Litani) - if they want to," hesaid, hinting that Syria might want to avoid such obvious muzzling of its Palestinian allies.

Nor did Gur appear to welcome the Syrians moving down to the Litani once the U.N. buffer force was in place there. Without further explanation he said, "If the Syrians want to cooperate from areas they control (now) there won't be any fedayen (guerilla) activity."

He praised the Syrians for not overacting to the Israeli invasion, whose vanguard units were no more than seven miles from Syrian troops.

"We never thought the Syrians would react. Fortunately, they understood enough," Gur said. "We did everything possible not to intrigue them."

He apparently meant that Israel had bent over backward not to provoke Syria or allow its own actions to be misunderstood.

Accompanied by two generals and other staff officers, Gur had arrived in a brown command car. He appeared relaxed. In contrast, the front-line troops who dotted the rocky ridgeline in infantry and armored positions were edgy.

The journalists had driven up the mountains from the battered port of Tyre following two representatives of the Geneva-based International Committee of the Red Cross.

Gur defended his invasion plan in the face of critics who have questioned its motives as well as its efficiency.

He reiterated that he had not sought to capture Tyre - which, along with a two-by-eight-mile salient, is the only Palestinian-head territory south of the Litani - in order to avoid casualties among his troops and the port's civilians.

Told that Israelo heavy artillery, naval gunfire and air strikes had had their effect, he said "You don't know what it is to hit in war. We didn't want to do it."

Asked if the Israel invasion had helped solve the Lebanese crisis, he said, "Sometimes you need a crisis to solve problems."

Only Palestinian guerillas would be prohibited from coming back to the Israel-occupied zone, he said: The Israelis are not going to get involved in trying to establish which Lebanese collaborated with the guerrillas.

"All Lebanese civillians can come back" to Israeli-occupied areas, he added. "Much more than hundreds are coming back." Few could be seen yesterday, however.

Judging by the experience of two women and a man who unsuccessfully sought to enter a nearby village for a member of their family and return to Beirut, Israeli policy does not allow free passage in both directions.

Apparently the Israelis are worried that such travellers could provide military information for the Palestinians. Gur, insisted however, that there had been "no ambushes for the time being," despite guerrilla claims of behind-the-lines raids.