Carrying a U.S.-made M16 assault rifle and dressed in U.S. Army camouflage fatigues with Ranger and Airborne patches, Isa Abdullah Ali makes no attempt to hide his nationality.

A tall, black American veteran of the Vietnam War from Washington, D.C., Abdullah says he is a fighter in the Lebanese Shiite Moslem militia called Amal, meaning hope. As far as is known, he is the only American fighting alongside Palestinian guerrillas and Lebanese Moslem militiamen against Israeli forces surrounding West Beirut.

Whether soldiers of faith or fortune, numerous foreigners have joined the battle against the Israeli invaders. But most of them are from other Arab states or Moslem countries such as Iran, Pakistan and Bangladesh. Only a handful are Westerners, including a couple of members of the Irish Republican Army recognized by correspondents.

For his part, Abdullah insists he is fighting for his faith, a militant brand of Islam that he says has instilled in him a "belief in martyrdom." It is his ambition, he says, to die in a battle to recapture Jerusalem from the Israelis.

So far, according to the 25-year-old American, the service of his faith has taken him to Afghanistan, Iran and Lebanon. Before that, he said in an interview today, he worked in the Iranian Embassy in Washington, serving the Shiite Moslem revolution of Ayatollah Ruhollah Khomeini.

Abdullah is known to Washington law enforcement officials because of his association with David Belfield, also known as Daoud Salahuddin, who has been charged with the murder of Ali Tabatabai, a former Iranian Embassy official and anti-Khomeini activist, in Bethesda in July 1980. Belfield signed as a witness on Abdullah's passport. Law enforcement officials questioned Abdullah and those who knew him in Washington, and determined that he was not involved in the Tabatabai assassination.

Abdullah said he was in Afghanistan at the time of the Tabatabai murder, returning to the United States in August 1980 because of illness after fighting for one month alongside Afghan Moslems resisting the Soviet invasion of that country.

Nevertheless, "after the assassination of this criminal Tabatabai," Abdullah said, he came under pressure from the FBI because of his links with Belfield, who is still being sought for the murder.

Abdullah said he left the United States for Lebanon in December 1980 and stayed here until October 1981. Then, he said, he went to Iran for eight months, returning to Lebanon three days before the June 6 Israeli invasion. He declined to specify what he did in Iran and Lebanon before the current war.

Friends have said that Abdullah has been active in training Amal soldiers, but he refused to confirm this "for political reasons."

"When advice is needed, I give it," he said. "When it's not, I'm a sniper."

Abdullah claimed to have killed at least nine Israelis with his M16, using a 500-millimeter telescopic sight. He said he covers "basically all the southern front"--the Palestinian refugee camps and Lebanese working-class districts near the Beirut airport--and roams among the positions of various guerrilla groups to "find a target."

"Even if our enemies the Iraqi Baath party militia can show me a target, I hit it," Abdullah said. He added that sometimes guerrilla groups specifically ask for him, "but most of the time I find a target on my own."

Draped with war paraphernalia including a canteen, binoculars, a dagger, a U.S. Army helmet and extra bullet clips for his M16, Abdullah said he had dropped out of school when he was 14 to join the American Army using forged identity papers. He said he became a Ranger and spent four months in Vietnam in 1972, but never saw combat there because of President Nixon's program to reduce U.S. involvement in the war. Abdullah said he spent most of his three-year tour in South Korea.

Shortly after leaving the Army, he said, he converted to Islam, initially joining the Sunni sect for two years before becoming a Shiite. Abdullah, who speaks some Arabic, said he has been a Moslem for 10 years. But he declined to give personal details of his past, including his former name. "That individual is dead now," he said.

In Washington, law enforcement officials said Abdullah was born as Clevin Raphael Holt. They said that his mother and stepfather are Peola and Kenneth Spurlock of Washington, and that his wife is Rimaah Ali, the former Maria Justine Garcia-Granados.

Abdullah said there is nothing to prevent him from going back to Washington, but "I really have no desire to return to the States."

"As far as the Lebanese people are concerned, I'm not a foreigner," he said as he sipped a Pepsi at a table by the empty pool at the Commodore Hotel in West Beirut. "This is my home, and my home is being destroyed."

"I'm quite sure that sooner or later I'm going to get killed," he added philosophically. "Where the end is only my God knows. When the Iranians reach Quds the Arabic name for Jerusalem , I ask my God to give me martyrdom. Once Palestine is free, I have no desire to stay in this life any longer."