Israel's plan to make use of a renegade Lebanese Christian Army officer to keep the peace in its proposed 25-mile-deep buffer zone across southern Lebanon already appears to be in danger because of this fragmented nation's intractable sectarian feuding.

With Israeli might and blessings to back him up, Maj. Saad Haddad has quickly moved his militia from its border enclave far northward into the heartland of Lebanon's Shiite Moslem population in a bid to fill the vacuum left by Israel's ouster of the Palestinian guerrillas from their strongholds there.

But his heavy-handed tactics and declared intention to impose his authority on the Shiites is already stirring resentment and sowing the seeds for more communal fighting. The Shiites are the smaller of the two great divisions of Islam. The schism between them and the Sunnis, who are considered the orthodox Moslems, arose out of different beliefs regarding the legitimate successor to Mohammed.

Everywhere along the roads and on the buildings and walls of villages in this mountainous Shiite region, the green cedar emblem of Haddad's "Free Lebanon" can be seen freshly painted.

His Israeli-armed militia is busy trying to absorb the Shiites' own much larger organization, Amal, while the major himself makes no bones about his objective--planting the flag of "Free Lebanon" all the way to the Awwali River north of the coastal town of Sidon.

But the Amal leader in Habbouch, Abu Ali Rumani, has other thoughts.

"Haddad should go back to the Lebanese Army," he said in an interview. "If he only wants a state for himself alone, we will not support him. If he works for all of Lebanon, then we will support. We want only one Lebanon, not two."

In an interview across the border in the Israeli military hub of Metullah, Haddad, 44, boasted that his former tiny border enclave, which he dubbed "the republic of Free Lebanon," now is 30 miles deep and almost 25 miles wide with more than 600,000 people--mostly Shiites--living in his domain.

Relaxed and feeling largely vindicated with respect to the highly controversial alliance he forged with Israel when he broke away from the Lebanese Army in 1976, Haddad remarked, "It looks like I was not a traitor after all. Now people are saying nice things about me. That's life. A very funny life. But that's life."

Haddad insisted that the reception given his "several thousand" militiamen by the Shiites had been "generally very enthusiatic" and that there was "no tension" between them and his Christian followers.

"They consider us as a dream come true," he said, adding, in a reference to the Israeli attack that drove the Palestinian guerrillas from the highland region, "They didn't expect their villages to be free again."

Haddad said he already had toured his enlarged fiefdom, given to him symbolically by Israeli Prime Minister Menachem Begin at a "hand-over ceremony" at the nearby Beaufort Castle shortly after its capture from the Palestinians in the first days of the war.

"I told them the Shiites there was no more need for Amal or its militia since there is now an equality of citizens," he said. "I told them they could join our Army as individuals but not as Amal members. If they don't join our Army, they have to lay down their weapons."

Amal leaders in this Shiite redoubt of 12,000 outside Nabatiyah make Haddad's grand design to eliminate Amal and create one big united southern Lebanon under his authority seem likely to prove little more than a dream.

Sitting in the salon of his pockmarked hillside home, the Amal leader, Rumani, made it clear that he does not intend to bow to Haddad's will.

"We have to have one republic, one state, one president and one people here in Lebanon now," Rumani said, repeating faithfully the line that Amal's national leader, Nabih Berri, has been taking in inter-sectarian political negotiations already under way in Beirut.

The stocky carpenter, a constant smile on his unshaven face and a twinkle in his eye, told in bits and pieces the story of what Haddad and his militia had been doing in this area since the Israeli invaders cleared the area of the Palestinian guerrillas.

Haddad, he said, had sent his men to all the Shiite village leaders and told them he wanted 10 men from each village to form a "home guard" under the control of his own militia.

Some villages gave him volunteers and others did not, he said. In some cases, individuals who were communist-oriented took advantage of the offer and presented themselves for arms, their political persuasion unbeknownst to Haddad.

"But we refused," said Rumani. "We said we want only the national Lebanese Army to defend our village."

While the Israeli Army had come to Habbouch and collected the guns of the Amal militia, Rumani insisted the men of the village were still ready and able to defend it "if Haddad makes trouble."

Later, he took his visitors on a tour of his house, pointing out which pockmarks were the result of Palestinian bullets and which of Israeli ones. None was recent, he said, and most dated back to the first 1978 Israeli invasion of southern Lebanon.

"In 1978 when the Israelis came here, all the people went away except Amal and the old people," he said. "Amal stayed only to defend them. We had no problem with Israel but the shelling was on us, not the Palestinians. Now it is better."

Haddad's hopes of being able to displace Amal and dissolve its militia seem largely dependent on Israeli will and military backup. So far, there is every indication that Israel is allowing him, even encouraging him, to expand his authority northward. It is providing him with FN and captured AK-47 rifles to arm his new Shiite militia recruits and Begin has given him his stamp of approval.

But whether Israel will continue this policy if warfare breaks out remains to be seen. Israel would face the dilemma of choosing between a pliable, if personally unstable, minority Christian ally and a politically far more important but unpredictable militant Shiite movement.

The chances of Israel's reconciling Haddad and Amal and thus avoiding the dilemma do not seem good, however. Already, Berri is complaining about Haddad's highhanded methods in dealing with some of his local offcials in the south.

While Amal did not oppose the Israeli invasion here in the south, its militiamen fought side by side with the Palestinian guerrillas against the Israelis around Beirut. The potential for Amal's turning against the Israelis down here is thus considerable.

There is also the potential for Amal's turning Haddad's Shiite followers against him.

This perhaps explains why Haddad says that the new "home guard" he has set up in Shiite villages is only "temporary" until the danger from guerrillas passes.

Then, he says, the Shiite militamen must turn in their arms or join his Army.