United Nations peace-keepers, long accustomed to playing the thankless man in the middle in the tangled politics of southern Lebanon, are finding themselves hard pressed as Israel's withdrawal from the area turns into a bloody confrontation with Shiite Moslems.

Symptomatic of the new tensions were two recent incidents. In the first, French troops of the 10-nation, 5,900-man force were roughed up as they sought to stop Israeli soldiers, responding to an attack on one of their patrols, from destroying houses in a Shiite village. The other incident involved a near shoot-out at the Qasmiyeh bridge over the Litani River on the coastal highway when an Israeli infantryman fired at French guards protecting a routine U.N. convoy to prevent it from moving north to Beirut.

The incidents were not judged serious in themselves, although in the first case a formal protest was lodged with Israeli authorities. But they recalled the almost constant bad relations between Israel and the lightly armed U.N. Interim Force in Lebanon from its inception during the Israeli invasion of the south in 1978 until last summer.

The participating nations -- at present a Swedish medical team and an Italian helicopter unit plus infantry from Fiji, Finland, France, Ghana, the Netherlands, Ireland, Nepal and Norway -- are responsible for a 430-square-mile area three to six miles north of the Israeli border stretching from the Mediterranean to the flanks of Mount Hermon.

UNIFIL also has jurisdiction under U.N. Security Council Resolution 425 for the roughly 100-square-mile border strip. But in practice the area was controlled even before Israel's 1982 invasion by the Israeli Army operating almost openly alongside the thinly disguised formal presence of its mainly Christian Lebanese allies.

UNIFIL's troops in the past have detained, disarmed but released the Lebanese Christians allied with Israel and their foes, the Palestinian guerrillas and their Lebanese allies.

UNIFIL's mission, to help the Lebanese government establish its sovereignty down to the border as well as to monitor Israel's withdrawal and help maintain peace and security in the south, became increasingly difficult, however, as the Palestinian-Israeli confrontation heightened tension on the border.

Before the 1982 invasion of Lebanon, Israel complained frequently that UNIFIL had been ineffective in preventing Palestinian guerrillas from establishing military bases in its part of southern Lebanon that put Palestinians within artillery and rocket range of settlements in northern Israel.

Today, as in the past, UNIFIL can do little to stop belligerents determined to shoot at each other, but their contacts with all parties and sometimes their presence has been enough to prevent the use of heavy weapons, for example, during this period of heightened tension.

Israel's long distrust of UNIFIL changed last summer with the inauguration of Prime Minister Shimon Peres' coalition government.

Suddenly, Israel found virtue in UNIFIL, trying to persuade the United Nations to move forces north to the Awwali River defense line to provide cover for its own Army's withdrawal and to remove the peace-keepers from their positions near the Israeli border.

If UNIFIL were to move north, observers noted, Israeli forces would have had a freer hand to intervene south of the Litani River.

It is in that area that UNIFIL, since its inception, has attempted more or less successfully to contain the level of violence between the Israelis and their Lebanese allies on the one hand and first the Palestinian guerrillas and now Lebanese Shiite Moslems on the other.

Syria vetoed Israel's plans for UNIFIL to move north, apparently to ensure that the Israeli withdrawal from Lebanon would be as painful as possible and to signal its refusal to cooperate with the Israelis.

This has left the small strip of southern Lebanon with a volatile mix of angry Shiites, Israelis increasingly determined to suppress attacks against their withdrawing forces and U.N. forces, once again, caught in the middle.

Political observers are convinced that the Israeli crackdown is potentially counterproductive for several reasons. First, signs are emerging that Israel's get-tough policy is serving to radicalize the Shiites further.

To the extent that anything approaching an overall Shiite command or policy exists, the fight against the Israelis stops at the border. But the Shiite resistance appears to be far from coordinated and thus is hard to control.

Shiite village leaders interviewed during a recent three-day visit to southern Lebanon said that the Israeli crackdown was making them consider taking the war to Israel proper, "making their civilians suffer as they have made ours suffer," as one put it.

Israeli officials, from Gen. Uri Orr, the northern command chief, on down, have sought to justify the crackdown by arguing that only getting tough with the Shiites now will stop them from carrying their fight to Israel proper.

More worrying to UNIFIL officers is the possibility that Israel may seek to expand the old border region under the control of its Lebanese allies into a uniform six-mile-deep security belt taking advantage of terrain features and road networks denied them until 1982.

Such a six-mile zone was widely believed to be Israel's goal during its 1978 invasion of southern Lebanon. But it was thwarted by quick U.S.-led diplomacy at the U.N. Security Council setting up UNIFIL.

A UNIFIL source said, "If the Israeli Army stays on in the UNIFIL operations zone despite Shiite opposition, then UNIFIL's position caught between them would be very difficult, indeed untenable." The zone snakes its way 35 miles from the Mediterranean to the Syrian border at varying distances from the Israeli border.

Israel is reliably reported to be considering taking advantage of high ground by slicing into the UNIFIL zone starting at the Mediterranean, then going north inside the zone to Tibnin, and then across the Litani River, keeping the strategic line-of-sight position at Beaufort Castle but leaving the Shiite market town of Nabatiye outside its direct or indirect control.

The Israelis say their withdrawal plan includes creation of a vaguely defined "security belt" in southern Lebanon in which they reserve the right to operate their own units as well as those of the Israeli-backed South Lebanon Army.

Such a perimeter would allow them to hang on longer to the Christian areas around Jezzin still farther north and the radar position atop Mount Baruk in the Mount Lebanon chain. It would also exclude the more militant Shiite villages northeast of the port of Tyre. But analysts worry that Shiite militants simply will start harassing the Israelis anew once the Army pulls farther back.