Israeli Chief of Staff Gen. Mordechai Gur warned yesterday that his troops would strike beyond their self-imposed six-mile limit in southern Lebanon unless Palestinians stop rocketing and shelling Israeli villages.

Gur said over Israeli radio that Israel "wants very much" to stay within the six-mile deep buffer zone it has been trying to create. "Buf if, for instance, Katyusha [rocket] and cannon fire on our settlements continues, and if terrorist fire doesn't stop, we won't hold ourselves back."

As if to reinforce Gur's warning, an Israeli Army spokesman in Tel Aviv said Israeli troops had occupied the village of Tibnine, eight miles from the Israeli-Lebanese frontier.

Israeli military sources said yesterday that a barrage of about 20 heavy artillery shells hit in northern Israel, sending residents of border communities scurrying to their air raid shelters. They said the shells had apparently been fired from north of the security belt. There was no report of casualties.

The Israelis have clamped heavy censorship on reports of the fighting in southern Lebanon, but the military has said that shelling and rocket attacks into northern Israel prior to yesterday had left two civilians dead and 12 wounded since Israel launched its drive four days ago.

Reports from Lebanon indicated that the Palestinians, badly routed in the first days of the massive Israeli incursion, were regrouping and fighting with guerrilla tactics.

Washington Post correspondent Jonathan Randal reported from Tyre that Israeli planes bombed the Rashideyeh Palestinian refugee camp south of the largely deserted Lebanese port city. Two Israeli gunboats sitting offshore continued a naval bombardment of the region.

Palestinian forces in the region have taken heart from the fact that they have survived the Israeli onslaught, Randal reported, much the same way that Egypt has pointed to its 1973 crossing of the Suez Canal, even though that venture ended up with U.S. intervention to stave off Israeli destruction of a large segment of Egypt's army.

Palestinian spokesmen in Beirut claimed that the Israelis had launched another major drive toward the Litani River - the so-called "red line" that demarcates southern Lebanon from the rest of the country and south of which Israel has refused to allow Syrian peacekeeping forces. There was no confirmation of the Palestinian report from Israeli sources, however.

Meanwhile, Syrian President Hafez Assad warned yesterday that "resistance would escalate" unless Israel withdraws its troops from southern Lebanon.

In his first public speech since Israel sent its troops into Lebanon in response to a Palestinian attack deep inside Israel, Assad said that Syrian airspace and land routes would be open to any Arab country that wants to supply the Palestinians.

Israel has said it does not seek a confrontation with the Syrian troops in Lebanon and there have been no reports of Syrians being involved in the fighting with the Israelis. Assad's speech yesterday gave no direct indication that Syrian forces would take a more active role against the Israelis, although he did say:

"Syria will remain the spearhead in confronting the invasion ... and this aggression will not force us to coexist with the enemy."

In an apparent allusion to splits within Arab ranks - particularly over Egyptian President Anwar Sadat's peace initiative - Assad added: "But we would like the instrument of Arab struggle to be more efficient. And this requires unity."

Assad's announcement about opening Syrian territory for resupply of the Palestinians appears to drop a previous stipulation that material sent by Iraq would have to be supervised by Syrian authorities. Iraq has offered to send the Palestinians any material they need.

In announcing the Israeli advance into Tibnine, the Israeli announcement said the move had come "at the request of village leaders," and noted that Tibnine had long been used as a Palestinian base. The announcement said that seven other villages in the region were also taken by Israeli forces yesterday.

A Reuter correspondent in the region said heavy Israeli air raids preceded the move by ground forces into Tibnini and that intense artillery duels raged between the Israelis and Palestinians in nearby hills.

More than 300 Lebanese and Palestinians have been killed and 400 wounded since the invasion, according to Lebanese Red Cross figures. Israel has said 15 of its soldiers have been killed and 57 wounded.

Reacting to reports of civilian casualties in Lebanon, Gur said that "civilians may have been hit," but said that the Palestinians have maintained guerrilla bases inside refugee camps "because they have long ago learned that we refrain from hitting innocent people."

"I'm sorry about every civillian hit," Gur said.

The latest spiral of Middle East violence began a week ago yesterday when Palestinian commandos seized a tourist bus on the road between Haifa and Tel Aviv in a violent raid that left 36 Israelis and nine of the 11 raiders dead.