Israeli infantry forces, now operating freely in southern Lebanon on a daily basis, today blew up two houses of suspected Palestinian guerrillas about five miles across the Lebanese border. The army warned that this policy of "deterrent actions" would continue.

The incursion into Lebanon was the fourth in as many days acknowledged by the government. It came a day after a clash in a U.N.-controlled area north of Metulla during which three Arabs were killed while allegedly attempting to infiltrate into Israel with weapons and explosives.

Operating with Israeli-supported right-wing Lebanese Christian militias led by Maj. Saad Haddad, the Israeli army patrols in the past week have demolished houses in southern Lebanon and killed five guerrillas. Several suspected terrorists have been abducted from Lebanon for interrogation in Israel.

The incursions have become so commonplace that army officials, in their descriptions of clashes across the border refer to Israeli troops there as "routine patrols."

Meanwhile, the headquarters of the U.N. peacekeeping force in southern Lebanon, alarmed by the growing instability in areas it purportedly controls, has voiced concern over the incursions by the Israel army and Haddad's troops, and has complained about "indiscriminate and unprovoked" shelling of U.N. positions.

Syria, which has 30,000 troops in Lebanon, has accused Israel and Haddad's forces of planning a major escalation of attacks against PLO% strongholds in Lebanon. The Syrian government newspaper Al Thawra said that Israeli Prime Minister Menachem Begin had sent messages to former Lebanese President Camille Chamoun and Phalangist leaders Pierre Gemayel urging them to "close ranks to cope with upcoming developments on the Lebanese arena."

Today's Israeli army incursion was in the Kabriha area near the northern finger of Israel. The army announced that during a "survelliance operation" in Lebanon, an Israel force blew up two houses after evacuating the occupants.

The Israeli Defense Forces will continue its policy of carrying out deterrent actions against terrorists and their accomplices," an army statement said.

Yesterday afternoon, three heavily armed Arabs were killed during what an army spokesman called a "routine patrol" in Lebanon. The army said the Arabs carried assault rifles, grenade launchers, explosives, and bullhorn louspeakers and wore explosive-filled "suicide belts."

An official said they carried mimeographed ransom demands indicating they had planned to take hostages in exchange for Palestinian prisoners. The leaftlets had blank spaces for the number of hostages to be taken, officials said.

Israel has charged - and the U.N. headquarters in Nagoura has denied - that 400 to 500 Palestinian terrorists are operating freely in the area controlled by the U.N. peacekeeping force. Israel has complained that the Palestinians infiltrate through U.N. positions in broad daylight without impediment.

The U.N. headquarters has shown growing impatience with the Israeli incursions and the actions of Haddad's units. Timur Goskel, U.N. spokesman in Naqoura, listed these incidents:

Militias opened fire Friday night on an Irish U.N. battalion near Brashit with a 22-minute barrage, during which nine heavy mortar shells hit the headquarters. There were no casualties.

An official of the Lebanese Ministry of Education who went to Naqoura Friday to organize high school exams was abducted by the Christian militias. She was released only after U.N. intervention.

In an attack Thursday in Majdel Silm, one villager was killed and two abducted. Two houses were demolished.

Sunday, an unidentified armored halftrack with eight soldiers shot its way throught a Norwegian U.N. checkpoint near Kfar Shuba. The halftrack withdrew when the Norwegians ordered up anti-tank reinforcements.

On July 7, militias entered a village in the Nigerian U.N.-controlled area, blew up a house and abducted a villager.

Responding to Israeli assertions that PLO units in U.N.-controlled areas have shelled and mined Christian villages, the U.N. headquarters said, "There has never been any shelling from (our) area at any time." CAPTION: Map, no caption, By Dave Cook - The Washington Post