The U.N. peace-keeping forces' headquarters in southern Lebanon was under virtual siege today by Israeli-backed militias led by renegade Lebanese Army Maj. Saad Haddad, who demanded U.N. guarantees that Palestinian guerrilla infiltration into his border enclave be stopped.

In a hastily called press conference at Bir Anit, on the Israeli-Lebanese border, Haddad today said he had ordered his troops not to fire at the U.N. headquarters. But he said that if the peace-keeping troops fire on the militias, "that will be the end of the United Nations in southern Lebanon."

Yesterday, Haddad's forces sealed off all roads to the U.N. headquarters in Naqura and blew up the only water pipeline to the base, trapping about 800 peace-keeping troops and 300 civilians who commute daily from the northern Israel town of Nahariya, U.N. officials said.

The U.N. Interim Force in Lebanon had said it had no intention of exacerbating the confrontation by trying to break out of Naqura, and said that it did not plan to negotiate with Haddad, whose forces are supplied and trained by the Israeli Army.

"As far as we are concerned, we are not part of any conflict. We are not going to make any proposals. We are doing the job we were told to do under the United Nations mandate, and it seems to me that if somebody obstructs our mission, they have the explaining to do," Timur Goksel, press officer for the peace-keeping force, said in a telephone interview from Naqura.

The confrontation began to build on Friday, when three of Haddad's militiamen were killed when their armored car drove over a land mine apparently planted by Palestinian guerrillas who had infiltrated the enclave the night before.

U.N. officials said that several hours later, a contingent of Haddad's forces made a cross-country dash in a tank and an armored personnel carrier and occupied a strategic hill near the village of At Tiri, about five miles north of the Israeli border.

U.N. officials said that after Haddad's forces seized the observation post, known as Hill 880, on Friday, Irish U.N. troops surrounded it and cut off the militia contingent's supply line. Following a series of negotiations and some disputes over Moslem dietary laws, the U.N. force began sending in food to the besieged militias, but warned that it would not withdraw until the militias abandoned the hill.

On Monday, U.N. officials said, Haddad's militias began blocking access roads to the Naqura base, a sprawling complex about three miles north of the Israeli border near the Mediterranean coast. The next day the militias cut off the Naqura headquarters' water supply.

For several weeks, Haddad has complained about infiltration of Palestinian guerrillas into his zone, accusing the U.N. force of failing to intercept them. On Nov. 6, Haddad announced he had resigned as militia leader, but he withdrew his resignation two days later.

An Israeli government official today noted that the Palestine Liberation Organization artillery emplacements north of the U.N. zone have been sporadically firing into Haddad's enclave, raising tension in the area.

Israeli Army Chief of Staff Lt. Gen. Rafael Eitan said today that although the PLO continues to build up its weaponry in south Lebanon, and while there have been occasional "incidents" involving Haddad's forces, the July 24 cease-fire is being "preserved in general."