A U.S. Marine Corps rifleman had an Israeli soldier in his sights and had to be ordered not to fire in the closest call yet in the recent series of U.S.-Israeli military confrontations in Lebanon, Pentagon sources said yesterday.

The incident is believed to have occurred near the Beirut airport Friday, the same day Defense Secretary Caspar W. Weinberger taped the Cable News Network's "Newsmaker-Saturday" program in which he said: "I am very worried about these incidents, and they are continuing."

The Marine rifleman, according to Pentagon officials, complained to his superiors that he had been trained to fire in self-defense, if necessary. These are the ground rules for U.S. troops in the multinational peace-keeping force in Lebanon, according to public statements of U.S. officials.

But the incident, although not yet known in detail, dramatized the murkiness of the rules of engagement in Lebanon and underscored worries expressed publicly by Weinberger and privately by military professionals.

The Pentagon announced Thursday that two confrontations had occurred within a day between Marines and Israeli forces at a checkpoint near the Beirut airport. One escalated to the point that an Israeli jeep "nudged" a U.S. Marine last Monday, according to the Pentagon. The Israeli military command has denied "any confrontation, physical or verbal," in the incident.

In Tel Aviv yesterday, United Press International reported an Israeli army charge that Palestinian guerrillas have escaped repeatedly through U.S. Marine lines after ambushing Israeli patrols.

The State Department said it had "no immediate comment" on the charge.

Weinberger did not mention the rifleman's confrontation in deploring the incidents but said they stemmed from Israeli patrols trying to use roads put under control of the multinational force of U.S., French and Italian troops.

Weinberger said the United States has lodged "vigorous protests" with Israel "because we are worried about an isolated incident of this kind that could grow into something much more serious."

Associated Press said it was told by Israeli military sources in Tel Aviv that Maj. Gen. Amir Drory, commander of Israeli troops in Lebanon, has reissued orders to his troops "to refrain from challenging" Marines in Beirut, and that Israeli troops had not crossed into Marine-controlled territory.

The Israeli sources also told AP that Marines built barricades in an area designated for Israeli control east of the Beirut railroad in the airport area. The sources quoted Marines as saying they were unaware of such designations.

Many Marine leaders view the situation in Lebanon as an accident waiting to happen and are becoming increasingly concerned about the Marines' seemingly open-ended stay. There are 1,200 Marines based at Beirut airport and 600 more aboard five ships offshore.

The Marines entered Lebanon for the second time last year on Sept. 28, and administration officials predicted in October that foreign forces would leave by year's end. The Marines' presence now seems indefinite as negotiations continue on withdrawal of Israeli, Syrian and Palestine Liberation Organization forces.

Civilian policy-makers, while acknowledging risks, contend that a continued U.S. military presence in Lebanon is vital for stabilizing the country and assisting in the withdrawal talks.

Before Marines were sent into Lebanon, Gen. David C. Jones, then chairman of the Joint Chiefs of Staff, warned publicly that sending U.S. troops into that volatile situation would not be a good idea.