The Israeli government has decided to intervene in the fighting in Lebanon if what it considers a hostile force attempts to move long-range weapons south of the Damur River, an authoritative source said today.

The source said the decision was made within the past week and reflected its increasing dismay at the number of Palestinian guerrillas fighting alongside Lebanese Druze militiamen in the Chouf mountains southeast of Beirut.

Israeli countermeasures are likely to involve hit-and-run attacks against selected targets launched from behind the current Israeli defensive line on the Awwali River, which is about nine miles south of the Damur, the source said. He said the Israelis would be reluctant to resort to air strikes against targets in Lebanon because this could give the impression that Israel was prepared again to get deeply involved in the fighting there.

"There is a Damur line," the source said, adding that the decision explains in part the apparent increase in the number of armored Israeli patrols north of the Awwali River. The patrols are not only "showing the flag" but scouting the terrain over which hit-and-run attacks may be launched, he said.

This account of Israeli intentions conforms with comments by Israeli military sources, who have also begun speaking in recent days of a "Damur line." These sources have emphasized that Israel's main concern is the possibility that Palestine Liberation Organization forces now fighting in the Chouf with the Druze could infiltrate farther south and pose a threat to the Israeli Army.

"No one said the Awwali is a static line," a military source said. "We will conduct operations to prevent the PLO from getting a foothold in the south. We will prevent them from getting below the Damur line."

The Israelis withdrew from the Chouf mountains on Sept. 4. This was followed immediately by the outbreak of the current fighting between Syrian-backed Druze militiamen and elements of both the Lebanese Army and the Christian Lebanese Phalangist militia.

The Israelis have warned Syria not to attempt to move troops into areas they evacuated, but otherwise they have kept their intentions regarding the current fighting vague. But in drawing its own "red line" along the Damur, Israel appears to have confirmed its dissociation from the fate of the Lebanese government of President Amin Gemayel and its return to a focus on its principal enemy, the PLO.

Informed sources said the Israelis made this clear immediately after the withdrawal when they ignored an "urgent appeal" from the Gemayel government for air strikes against Druze forces that were attacking the Christian town of Bhamdun, near the Beirut-to-Damascus highway. The appeal was relayed to Israel by the United States on Sept. 5 and was greeted by an icy silence in Jerusalem, the sources said.

According to Israeli military sources, between 1,000 and 1,500 Palestinian guerrillas are taking part in the Chouf mountain fighting. Other sources put the number of PLO forces in the Chouf at twice that figure.

Whatever the number, the Israelis are concerned that the PLO forces will get within range of attacking Israeli positions south of the Awwali, where there are now said to be fewer than 10,000 Israeli soldiers manning the defensive line.

The Damur River was among the lines the Israelis considered for their new defensive position. But according to sources familiar with the negotiations that preceded the pullback, Lebanon opposed it, arguing that use of the Damur as the new deployment line would split the Chouf mountains and complicate the task of attempting to maintain order in that region.

In the end, the sources said Israeli military planners decided the Awwali River line offered more protection for their troops and would still allow them to send armored patrols to the north to maintain a buffer zone for the Awwali line.

As the fighting in Lebanon has continued to rage, informed sources also described the Israelis as increasingly concerned about their relations with the Lebanese Druze and the impact of the fighting on Israel's 50,000-strong Druze community. The sources said the Israelis counted on the Druze to keep the PLO out of the Chouf mountains, but have become disillusioned by the growing Palestinian involvement in the fighting and begun to doubt whether the Druze will be willing or able to expel the PLO from the region.

There is concern here over growing restiveness among Israeli Druze. They have always been loyal to the country and are the only non-Jews who serve in the Army, where their talents as native Arabic speakers and their reputation as tough fighters are highly valued.

Yesterday, Israeli Druze held a demonstration outside the U.S. Embassy in Tel Aviv to protest U.S. military support for the Lebanese Army units fighting the Druze militiamen around Suq al Gharb.

Israeli Druze also held a news conference here today to deny that Palestinian guerrillas were aiding the Lebanese Druze. They said the United States had no right to intervene militarily against a small religious community "fighting for its existence, security and natural rights."