Palestinian guerrillas took the offensive in southern Lebanon this week in order to break a security belt designed to protect Israel from attacks across the Lebanese border, a Palestinian commando leader said here today.

The Syrian-supported week-long Palestinian assault has already cracked the belt and thwarted Israel's plan to create a secure border for itself on the north through a narrow buffer zone of towns held by rightist Christian forces it supports.

One of the most important links in the chain, Marjayoun, a garrison town two miles from here and six miles from Israel's border - is now three-quarters surrounded by Palestinian fighters who have its supply lines covered by heavy artillery and rockets.

Marjayoun, the first town Israel-supported Christians took when they started to form the security belt in October, is under heavy bombardment from Syrian-controlled artillery and rocket-launchers north of here, including some atop the ancient crusader fortress, Beaufort castle. Parts of Marjayoun can be seen burning as a result of the heavy shelling.

But the rightists were getting in some artillery licks today, too. Shells fell around the Palestinian headquarters here as Capt. Mousa Iraqi said that Monday's Palestinian offensive was designed to break the Israeli buffer zone.

[Israeli artillery fired at targets west of the Litani River toward Arnoun in southern Lebanon and into an area north of Marjayoun, according to Israeli military sources, according to Israeli military sources, Reuter reported from Tel Aviv. The Israelis shelled the targets after Palestinian shells landed near the Israeli town of Metullah and the settlement of Misgav Am, the sources said.]

Iraqi said the Palestine Liberation Organization decided to take the offensive in southern Lebanon after the Israeli-supported Christians took control March 31 of Taibe, a hilltop village two miles from Israeli that commands a string of five other border villages.

"That completed the security belt, and we could not allow that," said Iraqi, who commanded the Palestinian assault that won control here early yesterday.

Khiam, just four miles from the border, is the military barracks here. It also commands the supply route to Marjayoun, and Palestinian guns here prevent the Christians from getting reinforcements during daylight hours. They can use the road at night if trucks douse their lights, Palestinians here said.

TThe Palestinian attack, which started when the recapture of Taike, followed a secret meeting Sunday between two top Palestinian officials - Yasser Arafat and Abu Jihad, military commander in the south - and Syrian air force Maj. Gen. Naji Jamil, the deputy defense minister and a close confidant of Syrian president Hafez Assad.

Abu Jihad went to Damascus Wednesday to report to Jamil on the southern campaign.

The Palestinian assaults against rightist Christians obviously had the support of Syria, which for the first time took a role in the conflict in southern Lebanon. The shelling of Taibe Monday was done with Syrian weapons, including some rockets Palestinian organizations do not have, and Syrians began loosening their hold over the flow of military supplies to the Palestinians.

Syria is supporting the Palestinian offensive, a source in Damascus said today, because it does not want to lose what it feels is a strong negotiating card in any Middle East peace talks - the presence of Palestinian guerrillas in southern Lebanon and the threat of attacks against Israel.

A visit here today showed that the Palestinians - curbed in recent months by the Syrian-dominated Arab peacekeeping force in Lebanon - are still a potent military force.

Roads in this area were filled with Palestinian jeeps and trucks, and fighters were billeted both here and in the village of Ibl es Saji preparing to assault Marjayoun.

They laughed and joked as artillery shells from Marjayoun landed in the towns, but carefully herded a group of visiting reporters to a more secure shelter when one shell fell alongside the headquarters in Ibles Saqi.

From Marjayoun, the Christian forces can see up to the Palestinian-held villages and have clear shots at Palestinian vehicles bringing supplies and reinforcements here. The Palestinians have carved a dirt road into the hillside to give them more cover, but even so Christian artillery shells were able to land just behind a speeding jeep.

Palestinians said the Christians generally firre a shell at them every five minutes. But if the Christians spot any activity on the road or in town they intensify their barrages.

As a shell landed nearby, one Palestinian military commander assured reporters, "There is nothing to be afraid of. It is normal. Normal."

But thousands of residents have fled these villages, piling into decrepit cars with everything they can carry or walking to the road and begging for rides out. Cows and other livestock have been left to forage for themselves.