Israel's military believes that the contingent of Iranian volunteer guerrillas that is attempting to link up with the Palestine Liberation Organization in southern Lebanon is still in Syria and probably will remain there until Ayatollah Ruhollah Khomeini gives the group his personal approval.

The approximately 250 to 300 volunteers, Israeli military sources said, not only are regarded as a potential irritant by the Lebanese government headed by Elias Sarkis but, contrary to its public statements, are not being welcomed by the PLO.

Since the population of southern Lebanon is predominantly of the Shiite sect of Islam, the PLO fears it would lose control of its strongholds in the area if a significant Iranian fighting unit were allowed in, according to the Israeli sources.

"It would bring a new factor into Lebanon. It would raise the level of trouble between the civilians and the PLO," a high military source said.

Moreover, Syrian President Hafez Assad, burdened with his own domestic instability and challenges to his government, was said to be distressed about the presence of the volunteers.

[Arab sources in Beirut recently have told U.S. reporters that Assad does intend to restrict the movements of the Iranians, and reports of this have appeared in the Arab press.]

Traditionally, Syria has sought to accommodate Iran because of both countries' longstanding antagonism toward their mutual neighbor, Iraq. The military sources predicted that if Khomeini gave his official backing to the volunteer army, Assad probably would feel obligated to allow the unit to cross the border into Lebanon. The volunteers reportedly are at a PLO training base near Damascus.

But without such pressure, the sources said, Assad is not likely to risk massive Israeli retaliation in Lebanon, where Syria has 30,000 troops.

Israeli Defense Minister Ezer Weizman has said, cryptically, that if the volunteers enter southern Lebanon, they would become "acquainted" with the Israeli Army.

The PLO coolness toward the Iranian volunteers has caused some embarrassment within the organization, because of its close association with Khomeini's revolution. The PLO helped train and equip Iranian guerrillas during the revolution, and it has sought to strengthen its ties with the Islamic republic since Khomeini came to power.

An informed military source said today that, contrary to disclaimers by the U.S. State Department, Palestinians from Fatah, the military arm of the PLO, are among the guards at the U.S. Embassy in Tehran. Asked whether that assessment was based on Israeli intelligence, the source replied, "I am completely convinced of that."

In answer to reports from southern Lebanon that some Iranian volunteers had already arrived in Nabatiyeh, a PLO stronghold, the source said the reports had confused the volunteers with some Iranian soldiers who deserted the former Iranian battalion of the United Nations peacekeeping force in southern Lebanon. The deserters, numbering about a dozen, have been assimilated into PLO units, the source said.

Maj. Saad Haddad's Christian militias in southern Lebanon periodically have shelled Nabatiyeh on the basis of a war council decision to fire on any town giving shelter to the Iranian volunteers. The shelling started on Dec. 18, a few minutes after the pro-Haddad Voice of Hope radio broadcast in English, Persian and Arabic the rumors that Iranians had already reached Nabatiyeh.

The Israeli military sources also disclosed that Syria-based PLO guerrillas have attempted "10 to 12 times" in the past year to infiltrate into Israel by way of Jordan. The attempted border crossings had not been made public before.

The source said Jordan has "resisted" being used as a conduit for such infiltration, and said the crossings could strain relations between Jordan and the PLO.