In its first major retreat since 1985, a Lebanese militia supported by Israel withdrew from a strategic mountain enclave in southern Lebanon today as Islamic guerrillas harried them with gunfire and roadside bombs.

The pullout from the town of Jazzin represents a stinging defeat for the South Lebanese Army, a largely Christian militia force of about 2,500 men who are armed and paid by Israel, Lebanon's neighbor to the south.

Their departure is a triumph for the Iranian-backed fighters of Hezbollah, or Party of God, an increasingly efficient guerrilla force that has fought to expel both the South Lebanon Army and Israeli troops from the region for 14 years.

The militia commander, Antoine Lahad, withdrew south into the Israeli-occupied "security zone" of southern Lebanon because of what he called the "slow death" of his demoralized forces at the hands of the guerrillas. As the troops retreated, two militiamen were killed by bombs and a third was wounded.

The pullout from Jazzin could set the stage for a broader withdrawal of Israel's beleaguered forces, who are eager to end their two-decade presence in southern Lebanon. Israel occupies a nine-mile-deep buffer zone in southern Lebanon designed to protect its northern border.

Several dozen Israeli solders are killed each year in the zone, by ambushes, roadside bombs or mortar attacks, and the war has become unpopular in the Jewish state. Lebanese civilians have also been caught in the middle, as in 1996, when roughly 150 were killed in the Israeli air and artillery offensive known as Operation Grapes of Wrath.

Israeli Prime Minister-elect Ehud Barak, who will take office in the coming weeks, has promised to withdraw troops from southern Lebanon within a year.

Much depends on who fills the vacuum left by the South Lebanon Army's departure from Jazzin -- whether it is Hezbollah, which is widely admired in Lebanon as a patriotic resistance to the Israeli occupation, or the Lebanese government, which has been largely absent from the area for many years.

If Hezbollah moves into Jazzin, whose remaining population is almost entirely Christian, it would raise fears of vengeance killings against Lebanese seen as having collaborated with Israel.

If the Lebanese government moves to secure the enclave it would reassure Israel as it plans its own eventual withdrawal and would lower the chances of a bloodbath and cross-border attacks by Hezbollah.

U.S. diplomats, who have been monitoring the withdrawal, have quietly urged the Lebanese government to reassert its authority in Jazzin to calm the situation.

They are also watching signs from Syria, the dominant regional power in Lebanon. Syria, which controls the arms flow to Hezbollah, has used the Islamic guerrillas as a bargaining chip to recover the Golan Heights, which Israel has occupied since 1967.