MARJUYUN, LEBANON -- Israel and Syria, in a small but potentially significant sideshow to the Persian Gulf War, are effectively but unofficially cooperating in southern Lebanon with the common aim of neutralizing the pro-Iraqi militia forces of the Palestine Liberation Organization.

While refraining from any military action of its own against Iraq, Israel has carried out its most vigorous offensive in southern Lebanon in recent years during the past week, according to Israeli officials.

Responding to rocket attacks on a section of Lebanon that Israel holds as a "security zone," Israel's army and the local militia it maintains, the South Lebanon Army, shelled Palestinian positions for five days last week, carried out a major air raid, and launched a combined ground and helicopter assault near the southern town of Jazzin.

The rocket attacks and punishing responses broke a two-year truce in southern Lebanon between Israel and the 7,000 fighters of the PLO's mainstream Fatah group based in and around refugee camps at Tyre and Sidon. Israeli officials charge that Yasser Arafat, who heads the PLO and Fatah, ordered the attacks, which caused no injuries or damage, in an effort to open up a "second front" on behalf of Iraq in the gulf war. The PLO has denied this.

Israel, however, appears to have used the ineffectual rocket barrages as an opportunity to deal another political and military blow to Arafat's PLO at a time when it is already reeling from the consequences of its tilt toward Baghdad. In this aim, its unlikely, undeclared ally has been Syria, a member of the U.S.-led coalition against Iraq that has taken advantage of the Persian Gulf crisis to steadily expand its influence in Lebanon.

Less than a day after the Israeli offensive against Fatah ended Thursday, seven battalions of the Lebanese army, under the Syrian-controlled government of President Elias Hrawi, began deploying southward from Beirut to points north of Israel's self-declared security zone. The zone is a 325-square-mile strip of land that Israel has held along its northern border since withdrawing most of its army from Lebanon in 1985, three years after it invaded.

In theory, the Lebanese army deployment was part of a plan by Hrawi's government to assert control over all of the embattled country and displace its many militias this year. But the move to the south appeared to have been hastily arranged after the first rocket attacks two weeks ago, and it coincided with demands from Syrian spokesmen in southern Lebanon that the PLO cease its incipient military activity against Israeli forces.

The Lebanese units posted in the south so far, with a total of about 2,500 troops, are as yet no match for the well-armed Palestinian forces, experts said. But Israeli officials said they have already shown surprising assertiveness in their mission to take control of battle-scarred southern Lebanon and halt PLO activity.

In the four days since being deployed, the Israelis said, the Lebanese have set up checkpoints on major roads leading from Palestinian camps and other strongholds on the coast, and have disarmed squads of Palestinian fighters in at least two instances. The army also has taken over positions of the pro-Syrian militia Amal but has not yet challenged the other major force in the area, the pro-Iranian Hezbollah.

Local observers said no Lebanese army force has attempted to exert such authority over the many militias in southern Lebanon since 1975, when the civil war began. Commanders of the South Lebanon Army (SLA), the 2,500-man militia organized and financed by Israel to police the security zone, express enthusiasm about the development.

"The deployment is very good for us, because the mission is to avoid terrorist infiltration into this area," the SLA commander, Gen. Antoine Lahad, told reporters visiting here Sunday. "All parties support the deployment of the Lebanese army into the region, except the PLO."

Israeli officials are more circumspect, especially with regard to Syria's long-term intentions in the area. "It's maybe the beginning of the test. The test is whether the Lebanese army will be able to prevent terrorist activity," said Gen. Yossi Peled, chief of the Israeli army's northern command. Col. Raanan Gissin, an army spokesman, added: "The Syrians obviously want quiet in their back yard in Lebanon right now. But we don't know if this is temporary or permanent."

Syria's moves in southern Lebanon intrigue some Western diplomats, who have been hoping that its role in the alliance against Iraq could be parlayed into rapprochement with Israel after the war. Officials of Israel's right-wing government, which has been cool to such suggestions, contend the Syrians are merely engaged in a tactical maneuver.

However, Israeli army officials acknowledge that, in extending their control into southern Lebanon, Syria and the Hrawi government have carefully avoided provoking Israel. Operations by Syrian-backed guerrilla groups in the south against Israeli targets, which surged last fall, have abruptly ceased. Last week, Syria's own troops stood by as Israel's elite Golani Brigade went into action within a few miles of their positions.

"The PLO is in the worst position it has been in for years in Lebanon," Israeli army spokesman Gissin said. "It has lost all of its traditional allies and made some new enemies."

If successful, the Lebanese army deployment eventually could also undermine Israel's own presence in the south, analysts said. Although it has established a substantial administrative presence in its zone in Lebanon, Israel has long insisted it holds the territory only to protect its own northern settlements and will give it up as soon as there is a government in Lebanon that can effectively control the area.

SLA commander Lahad has declared allegiance to the Hrawi government and says his force has "under the table" contacts with the Lebanese army. Still, Lahad and Israeli commanders here have begun to echo the sentiments of Israeli Prime Minister Yitzhak Shamir's government, which seems unwilling to accept a Lebanon controlled by Syria even if it can enforce the same quiet along the Israeli-Lebanese border that exists on the Israeli-Syrian frontier.

"It's not enough that it will be quiet here for a few months," Peled said. Israel "would also have to come to an agreement with the government of Lebanon."