The commander of the Israeli-supported militia in southern Lebanon warned today if Palestinian guerrillas again shell his border enclave, his forces will "retaliate hard" despite a U.S.-engineered truce.

Exchanges between the secessionist enclave and guerrilla forces could further endanger the fragile cease-fire between Israel and the Palestine Liberation Organization, which already has been violated and repudiated by one PLO splinter group.

Maj. Saad Haddad, the renegade Lebanese militia leader, said on Israeli radio that he would disregard any Israeli objections to retaliation and order his artillery to open fire if the Palestinians fail again to respect the cease-fire worked out Friday by U.S. special envoy Philip C. Habib.

"Now, I took the decision. If they will shoot again I am going to retaliate hard, even if there is an objection to that," Haddad said. "It's our lives. We care if we are always going to give our neck for them. It can't be."

The Palestinians fired a salvo of Katyusha rockets from Nabatiyeh into Haddad's enclave tonight, the Israeli Army Command said. Neither Haddad nor the Israeli Army returned the fire.

Earlier in the day, the Israel Lebanon border had been quiet. But Israeli reconnaissance planes flew twice over Beirut, drawing Palestinian antiaircraft fire and charges of truce violations. PLO offices in the coastal city of Sidon also reported high-altitude reconnaissance flights.

["We consider it a violation but we will act with restraint," said a PLO official in the Lebanese capital, The Associated Press reported. It quoted the Palestinian news agency, WAFA, as calling the overflights "defiance" of the cease-fire.]

The Israeli Army command had no comment.An Army spokesman said, "Anyway, there is no connection between the agreement and reconnaissance flights."

Three times Friday night and yesterday morning, artillery shells and Katyusha rockets fell inthe narrow Haddad enclave that stretches 60 miles from the Mediterranean to the foothills of Mt. Hermon. The principal target was Marjayoun, the site of Haddad's headquarters and a strong point of Israeli armor and artillery deployed in the enclave.

The Libyan-backed Popular Front for the Liberation of Palestine -- General Command, a PLO group headed by former Syrian Air Force officer Ahmed Jabril, claimed responsibility for the shellings and said it will not honor the cease-fire arranged between Israel and the PLO through Habib, the United Nations, Saudi Arabia and the Lebanese government.

PLO spokesmen in Beirut earlier releassed a reprimand to Jabril's group and vowed that Arafat's orders will be enforced. The spokesman for U.N. forces in Lebanon, Timor Goksel, told Israeli television tonight that Arafat's commitment to the United Nations covered all groups, according to the Reuter news agency.

"This includes all Palestinian groups under the PLO umbrella, the non-PLO groups which are the rejectionist factions plus the leftist Lebanese," he said. "So he has committed himself on hehalf of all these."

In his interview on Israeli radio, Haddad said: "I am for the peace [but] I would like also that there be a real cease-fire, and I will accept a real cease-fire. But I don't like this fore from one side only. So, I did not answer because I wanted to show the world that they are the aggressors and because the United Nations used to acuse us of always being the aggressors."

The U.N. peacekeeping force in Lebanon traditionally has regarded Haddad's 500 former Lebanese Army regulars and 1,500 predominantly Shiite Moselm irregulars as a surrogate of the Israeli Army, which pays and supplies Haddad's forces and also maintains its own permanent presence inside the enclave.

One of Begin's aides, speaking half facetiously, commented today that "Marjayoun is in Israel."

There has been an increasing debate within Israel's government and the defense establishment over whether the cease-fire will undermine Israel's security interests in the long run, giving the PLO an opportunity to regroup after two weeks of debilitating air and ground strikes by Israeli forces.

The Israeli Army chief of staff, Lt. Gen. Rafael Eitan, has said he has no doubt the guerrillas will use the cease-fire to regroup and that once they are "back on their feet" they will resume attacks. Similarly, Maj. Gen. Avigdor Ben-Gal, commander of the northern region, called the cease-fire a "pause" after which hostilities will resume.

[The PLO chairman, Yasser Arafat, said his forces are determined to continue organizing in Lebanon during the cease-fire. "Definitely we have the right to organize ourselves," he said on the interview program "Issues and ANSWERS" (ABC, WJLA].

The Israeli military's assessments, beside illustrating the ease with which Israeli officers believe they can contain the guerrillas if not fettered by political considerations, appeared to underscore doubts said to ahve existed in the defense establishment while the cease-fire was being considered.

Agriculture Minister Ariel Sharon, who is expected to be named the new defense minister, reportedly was the only Cabinet minister to oppose the cease-fire proposal brought by Habib. However, top-ranking military officers were said to have warned Begin that the guerrillas would only use the lull to transport men and arms to southern Lebanon for new attacks.

Foreign Minister Yitzhak Shamir, when asked about the military's concerns, replied, "Well, past experience teaches us that cease-fires of this type must be regarded with a certain amount of doubt. But in spite of this, we want to hope that this cease-fire will hold, because it is in itself a positive event and, I would say, an accomplishment."

Meanwhile, Israel has asked the International Red Cross to investigate what spokesmen called the "possibility" that Palestinian guerrillas in Beirut are holding an Israeli citizen.

Last week, the PLO said it captured an Israeli soldier during an attempt to land a raiding party near the Zahrani River on Monday, but the Israeli Army command denied any of its troops were missing. Subsequently, the PLO produced a man identified as David Manache, who said in a radio interview in Beirut that he was an Israeli soldier.

An Army spokesman said today it is possible to PLO is referring to an Israeli civilian named David Aziz Moncherie, who was given a medical discharge from the Army 10 years ago because of "psychological maladjustment" to the military. Moncherie was classified as a "military invalid" and following his discharge had often disappeared for long periods, the Army spokesman said.

The spokesman, when asked if Manache was a Hebrew version of Moncherie, replied that the records carried only the latter, but that the man's photo and identity number were "similar."

The Army said Moncherie left the country seven months ago, ostensibly for Greece, and had not been heard from since.