Five U.S. marines were wounded superficially by a grenade today in the first direct attack against the 1,200-man force since American troops took up positions at Beirut International Airport last September.

The attack on the 12-man marine foot patrol occurred less than 12 hours after eight Italian soldiers also serving in the multinational peace-keeping force were wounded in two separate firefights late last night. They were the first Italian casualties due to hostile fire. Three of them were seriously injured, including one who lost a foot.

Several hours after the attack on the marines, an anonymous caller told the French news agency Agence France-Presse that a group called Jihad Islami, or Islamic Holy War, was responsible. Marine commander Col. James M. Mead told reporters that his men did not return fire because they could not see their assailant.

"We will not allow a single act of terrorism to stop us from doing our mission," Mead said. "Our patrols must continue and will continue."

In Washington, White House spokesman Larry Speakes said that the attack "appears to be an isolated incident" and that it would not affect U.S. policy in Lebanon.

"The amazing thing is not that this happened," a western diplomat said, "but that it hasn't happened much earlier." He was expressing the widely held view here that the multinational force, especially the U.S. contingent, constitutes a juicy target for the wide variety of armed men who either oppose U.S. policy in Lebanon or are tempted to cause trouble hoping to pin responsibility on their political adversaries.

The new attacks on the multinational force took place against a background of increasing attacks against the Israeli Army in Lebanon. Hardly a day has gone by in the past week without the Israelis suffering casualties.

Two Israeli officers were killed Wednesday when their jeep struck a mine on the Beirut-to-Damascus highway near Kahhalah, three miles east of the capital, the Tel Aviv military command reported.

The incidents involving both the Americans and Italians took place in Beirut suburbs north of the airport, an area inhabited mainly by Shiite Moslem Lebanese.

The anonymous caller said that the attack on the marines was against "the American occupation army." Pro-Iranian partisans of Mohammed Hussein Moussawi, a former leader of the Shiite Amal militia, have used similar language in the past to describe the multinational force.

The Lebanese government has issued arrest warrants for Moussawi and his principal lieutenants after armed men loyal to him killed six Lebanese Army soldiers in a raid early this month near the Syrian border.

Lebanese Army troops cordoned off the area near the grenade attack against the marines and detained 350 men, Lebanese sources said. Most of the men later were released, they said.

Italian military sources had wondered last night if their 1,000-man force was being singled out after the ambushes. After the attack on the marines, however, they suggested that the attacks were coordinated and directed against the multinational force as a whole.

Also serving in the force are 2,000 French paratroopers and just under 100 British guardsmen who arrived last month.

The only previous attacks against the multinational force were directed against the French contingent, which is considered to be the most active. It has small units spread out in many parts of the Beirut area. Three French soldiers were wounded, one seriously, in attacks on Jan. 29 and Feb. 2.

The assailants in those attacks were neither identified nor apprehended. Observers suggested that pro-Iranian Shiite Moslems may have staged the attacks in retaliation for France's unstinting support for Iraq in its 2 1/2-year-old war with Iran.

Lebanon in recent years often has served as the arena for settling scores involving Middle East powers and other states with interests in the area.

Observers theorized that the attacks against the Americans and Italians may reflect growing Lebanese frustration with the failure of the United States and its allies to persuade or force the Israelis to evacuate Lebanon.

Col. Mead said that one of the marines saw the grenade coming out of the window of the second story of a building in the Ouzai district, along the heavily traveled coastal highway. The five wounded marines were flown by helicopter to the U.S.S. Guadalcanal, a helicopter carrier that also provides medical facilities for the marines and 6th Fleet ships stationed off shore.

Mead said that the attack would not lead to any change in the marines' rules of engagement. Since their arrival, the marines have been forbidden to carry ammunition in their weapons. Only if fired upon are they authorized to load their weapons and fire back.

Mead said that he was pleased that a marine reaction force had reached the area of the grenade attack within 13 minutes of the assault. Lebanese Army troops also rushed to the scene and set up roadblocks.

The only death suffered by the marines here was sustained within hours of their arrival on Sept. 30. A corporal was killed while attempting to defuse an unexploded U.S.-manufactured cluster bomb unit left behind by the Israelis, and three other marines were injured.

In October, a car bomb exploded during a changeover of marine units, and a marine photographer was nicked by a fragment.