Heavy fighting broke out between Moslem militiamen and Israeli Army forces in southern Lebanon again today, resulting in the death of one Israeli soldier and a number of Lebanese guerrillas as Israeli armored units continued to search for two Israelis captured on Monday, U.N. peace-keeping officials and the Israeli Army command said tonight.

The most serious clash occurred this afternoon in the village of Srifa, 15 miles north of the Israeli border, when Moslem guerrillas ambushed an Israeli patrol, killing the soldier.

Israeli Army reinforcements responded first with small arms fire and then followed up with fire from artillery batteries, tanks and helicopter gunships, Timur Goksel, spokesman for the U.N. Interim Force in Lebanon (UNIFIL), said tonight by telephone from UNIFIL headquarters in Naqura.

Goksel said Israeli artillery batteries laid down a barrage of fire on roads north of the village in an apparent attempt to prevent the escape of Moslem militiamen.

Goksel said that a UNIFIL medical team had been sent into the village to check on reports that local residents suffered heavy casualties in the fighting.

The Israeli Army command in Tel Aviv said only that eight "armed terrorists in possession of combat materiel" had been killed in the exchange.

Goksel said that local residents reported that the Lebanese combatants were from the Shiite militia Amal, which is dominant in southern Lebanon, but that some guerrillas from the radical Shiite militia Hezbollah (Party of God) may have been involved in the fighting.

Israeli Prime Minister Shimon Peres said tonight on state radio that the Army will not leave Lebanon until it completes its search for the two missing soldiers.

However, sources in the Army command today began expressing fears that the two captured soldiers may have died immediately in Monday's ambush and that their bodies were taken away in the black Mercedes-Benz sedan that was seen leaving the ambush site. They said their fears were based on the amount of blood that was found in the bullet-riddled car in which the two soldiers had been riding.