Christian militiamen opened heavy machine-gun fire on the deputy commander of U.N. forces in Lebanon today at a border village where U.N. troops are locked in a tense showdown with the irregulars.

The Norwegian officer, Brig. Gen. Ole Nilsen escaped unhurt along with a senior U.N. political adviser, James Holger, and the commander of the Irish battalion in the U.N. forces, Col. Jack Kissane. Both were with Nilsen along with several other officials in an armored personnel carrier pulling into At Tiri just north of the border. [news agencies reported that Capt. Harry Kline, a member of the U.N. truce team that has been monitoring the Israeil-Lebanese border since the 1949 Arab-Israeli armistice, also was in Nilsen's party.]

The shooting underscored the increasing confrontation in these rugged, rock-strewn hills since Israeli tanks and armored personnel carriers moved across the border Wednesday to reinforce the Israeli-sponsored militia barring access to Israel by Palestinian commando squads.

Some of the Israeli armor pulled back out of view of U.N. observation posts today in what U.N. officials said was a gesture designed to reduce the impact of the Israeli incursion before an expected U.N. Security Council debate demanded by Lebanon. But Israeli soldiers and some Israeli armor remained within the 60-mile long free Lebanon border strip commanded by Maj. Saad Haddad, a secessionist Lebanese officer put in place and maintained by the Israeli Army to run the buffer zone where U.N. troops are prevented from patrolling.

"Don't think they left the enclave," a U.N. official said. "They just moved out of our sight."

[The Israeli Army announced tonight that it has begun "to withdraw from south Lebanon those of its forces who have completed their mission." The Army did not elaborate or indicate how many of its troops have been withdrawn, Washington Post correspondent William Claiborne reported from Jerusalem.]

Several Israeli soldiers were seen lounging at a checkpoint marking the end of the Irish-controlled U.N. zone at the southern edge of this village about one mile north of Bint Jbail and three miles north of the Israeli border. About 100 yards down the road, youths from the Haddad militia were throwing stones at Irish troops in a clash that at times verged on burlesque but dramatized the bad blood between U.N. forces on one hand and Israeli troops and Haddad's irregulars on the other.

About 200 Irish soldiers backed by armored personnel carriers and carrying FAL assault rifles spent several hours trading volleys of stones and smoke grenades with the youths, some of whom carried Kalashnikov AK47 automatic rifles. Even after the youths set fire to the tire of an Irish personnel carrier, the U.N. soldiers withheld their fire.

On both ends of Beit Yahoun, the Irish were the object of scorn. On one end, the Haddad militiamen called them a "whisky army" and ridiculed their efforts to carry out a two-year-old Security Council mandate ordering U.N. forces to control the area. At the other end, villagers prevented from reaching their homes because of the stone-throwing match shouted their wrath.

"You whore, what are you doing?" one villager shouted at the row of blue-helmeted Irish troops blocking the road. "They're been fighting among themselves for 50 years, and now they're coming to stop us from fighting?" asked another.

In At Tiri, several miles west of here, Irish, Dutch and Senegalese troops fired warning shots at the Sherman tank and half-track armored personnel carrier from which .50-caliber machine guns had fired on Nilsen's vehicle for about eight minutes.

The troops of the United Nations Interim Force in Lebanon have controlled the village with about 50 militiamen trapped inside since they repulsed a militia attempt to take it over last Sunday. Haddad's Israeli advisers have since been warned that a Dutch U.N. unit encamped on a ridge overlooking At Tiri will open fire with wire-guided missiles if the militia tanks fire their artillery again. U.S. sources said.

"At Tiri has become a matter of principle," a U.N. officer said.

The village offers a commanding view of the border region. But more important, U.N. officials feel, Haddad's irregulars have expanded their enclave at U.N. expense several times in recent weeks and the line must be drawn at At Tiri.

U.N. anger is especially high over the At Tiri incidents because an Irish soldier identified as Stephen Griffin was critically wounded in the head during Sunday night's exchange, and is said to be on the verge of death in a Haifa hospital with a rifle bullet lodged in his brain.

In addition, U.N. officials have complained bitterly of Israeli charges that the U.N. forces were negligent in allowing Palestinian terrorists to infiltrate its lines for the raid last Monday on a northern Israeli kibbutz. An Israeli soldier and two Israeli civilians, including an infant, were killed during the raid, as were five Palentinian commandos.

The main obstacle to doing a good job, U.N. officials feel, is the very Haddad militia that is sponsored and paid by Israel. "They say all the time that we are not doing our job," said an Irish U.N. officer here today. "And we spend all our time dealing with Haddad's people, the harassment and things."

At the same time, some officers in the 10-nation, 6,000-man U.N. force say their Irish colleagues have not had authorization from Dublin to be stiff enough in dealing with challenges from Haddad's irregulars. One officer said the response to today's shooting at Nilsen should have been a swift raid to take out the tank and half-track on which the machine guns were mounted.

The Israeli Army, which in U.N. eyes is the effective commander of Haddad's forces, would not want to be seen in out-and-out battle with U.N. troops, he said, and as a result would order the militiamen to back down.

"If we cannot stop this harassment -- and I mean Israeli harassment then we might as well all pack up and go home," he added.