Israel has increased its flow of ammunition and other military supplies to the South Lebanon Army in an attempt to buttress that force's defense of the Lebanese Christian town of Jezzin and its credibility as Israel's principal ally in southern Lebanon, informed sources said today.

The stepped-up supply effort was said to involve mostly ammunition for light arms, tanks and artillery to replenish the stocks of the force's units that are deployed in and around Jezzin, a Christian stronghold to which thousands of Christian refugees have fled in the last two weeks in the face of advancing Lebanese Druze and Moslem militias.

The South Lebanon Army, commanded by Brig. Gen. Antoine Lahad, a Christian, is supplied, financed and supported by Israel. It is designed to be the backbone of a system of local militias that are to police Israel's so-called security zone, a six- to l5-mile-wide strip of Lebanese territory along the border with Israel.

Israeli officials repeated today that they will not intervene militarily to defend Jezzin, from which the Israeli Army withdrew on April 29 and which is several miles north of the security zone. They have acknowledged in the past providing food, medicine and other "humanitarian aid" to the refugees in Jezzin, but it was the first time officials here also confirmed an attempt to shore up Lahad's military capability in the Jezzin area.

"We have a great interest in Gen. Lahad's credibility," a senior official said in explaining the Israeli military supply effort.

The official said Israel hopes the continued existence of Jezzin as a Christian stronghold defended by Lahad will make Lahad a hero to the Christians who live in the security zone, buttressing his credibility as a military leader and easing the task of recruiting new militiamen into the depleted ranks of his army.

Lahad's forces, which never numbered more than 2,000, have suffered a wave of desertions, especially by Druze and Shiite Moslems, as the Israeli Army has gradually pulled back toward the border. The force is now overwhelmingly made up of Christians from far-southern Lebanon.

An Israeli military official said Lahad's force now numbers between 1,200 and 1,500 men, and that several hundred of these are with Lahad in the Jezzin area.

"The whole idea is not to let them collapse," another senior official said. "We have made it clear to Lahad that we don't regard Jezzin as part of the security zone. Our backing for him does not apply to Jezzin. He says that what happens in Jezzin will reflect on the whole of southern Lebanon."

The official said Israeli analysts are divided over the likely impact on the security zone should the South Lebanon Army collapse and Jezzin fall to the Druze and Moslem militias. However, it is clearly in Israel's interest, according to observers here, for Lahad and his men to appear to be an effective military force, capable of defending the villages in the security zone from outside militias.

A further weakening of the South Lebanon Army, or its outright collapse, observers say, would probably lead to a larger and more active Israeli Army presence in the security zone than is now planned. The Israelis intend to withdraw from the security zone by early June, but have made clear that they will continue regular patrols and maintain observation posts there when neceesary in support of Lahad's men and other local militias allied with Israel.

Thus far, Lahad and his Israeli suppliers appear to be winning the gamble that Jezzin can hold. After driving thousands of Christians from their villages east of Sidon, the Druze and Moslem advance halted last week in heavy fighting around the village of Kfar Falous, about seven miles west of Jezzin.

There has been a lull in the fighting since then, with officials here describing the situation around Kfar Falous as a "standoff." Druze militia leader Walid Jumblatt said last week that he had no intention of attacking Jezzin, but Moslem militiamen have not indicated their plans. Meanwhile, some Jezzin residents who fled to the Israeli security zone as the fighting neared the town have returned to their homes.

Israeli officials were quick to give Lahad and his men credit for holding off the Druze and Moslem militias at Kfar Falous.

"History teaches that the Christians can defend themselves when they fight, but they don't want to fight," one official said. "They want Israel to defend them."