U.S. and Israeli officials met here today to resolve what the Americans said was "clearly an area of misunderstanding" leading to yesterday's confrontation between their respective military forces, and they agreed to establish a formal demarcation line to separate them.

Diplomatic sources said that civilian and military officials of the two countries together visited the spot where Marine Capt. Charles Johnson yesterday ordered three Israeli tanks to stop, then jumped atop one to halt their advance toward a jointly manned U.S.-Lebanese checkpoint. The officials agreed to deploy rows of colored barrels along the north-south railroad line that marks the perimeter of Marine territory.

In a news conference today, Johnson, who appeared embarrassed by his sudden fame, said he thought the repercussions from the incident, the latest in a series between U.S. and Israeli forces, were "a lot of fuss over not that big of a deal."

Describing the point at which he loaded his .45-caliber pistol and raised it in ready position, Johnson said "I just got on the tank and grabbed the guy and told him to stop his damn tanks."

Some Lebanese civilians are comparing the Marine derring-do to American fiction like "Dallas." There are indications here, however, that Johnson acted to ward off what could have been a bloody confrontation, not between the Marines and the Israelis, but between the Israeli Defense Force and the Lebanese Army.

As Col. Thomas Stokes, commander of the Marine contingent here, delicately put it the night after the incident, "His point was he was trying to avoid a situation where you would get a third party involved and a possible accident."

It is one of the ironies of the plight of the multinational force of Marines and French and Italian soldiers who came here to provide a powerful backup to the weak, splintered Lebanese Army, and help it to grow stronger so that it can extend the authority of the central government in Beirut, that it has become very aggressive before it has become very strong.

There are clear indications that what worried the Marines as the Israeli tanks rolled forward was less how the U.S. troops, their M16s slung over their shoulders, ammunition magazines in their pockets, would react as it was over what the Lebanese Army soldiers whose weapons were loaded might do in response.

Increasingly, multinational force officers are privately expressing concern over the sudden aggressiveness and emboldened behavior of the Lebanese Army, now that it has the backup of some of the most powerful armies in the world.

On Monday, Italian troops and United Nations observers intervened when they happened upon two Israeli pickup trucks carrying five soldiers who were believed to have accidentally turned out of their zone into Lebanese Army territory.

The U.N. observers felt the Lebanese soldiers were ready to open fire and intervened to avoid what they described as a "very serious incident."

Not all incursion into the wrong territory are accidental in this nation carved up into zones controlled by, at last count, 19 different armies.

In the case of the Marines and the Israeli forces, positions had been discussed at a formal meeting involving the commanders of the two forces and in discussions through diplomatic channels several times before then. The problem there has been a disagreement over what was previously agreed upon, but even though the precise dividing lines were left vague both sides were made aware of the other's positions.

Today's meeting, diplomats said, was to clear up any lingering misunderstanding.

Although the demarcation agreement was announced by the Israeli government in Tel Aviv, a U.S. Embassy statement afterward said, without giving details, that "today's meeting was held to resolve what was clearly an area of misunderstanding revealed by yesterday's incident and the misunderstanding was resolved."

In Washington, the Pentagon and the State Department declined comment about the reported agreement.

There are confrontations here, however, when the question of demarcation lines is not a matter of confusion.

Last Sunday in southern Lebanon, for example, United Nations sources reported the following encounter:

An Israeli officer driving through a checkpoint manned by French troops in the southern Lebanon U.N. peace-keeping force was stopped and told he could not enter the area armed. He left only to reappear a half hour later with an Israeli halftrack and proceeded to establish an Israeli checkpoint--in front of that which the French were manning.

For about an hour the Israeli checkpoint stopped all traffic coming down the road, including a car in which the acting U.N. peace-keeping forces commander was traveling.

When the Israelis finally decided to disband the operation, they nudged a French soldier with the bumper of the halftrack as they went through his checkpoint.

French troops then loaded their guns, and forced the Israelis to lay down their arms. They were subsequently taken off to a French commander who admonished them for more than an hour.