TUNIS, JAN. 15 -- The Palestine Liberation Organization today accused an Arab bodyguard once linked to the Iraqi-backed Abu Nidal terrorist organization of the assassinations late Monday night of two top PLO operatives.

Machine-gunned to death in a north Tunis villa were Salah Khalaf, known by his nom de guerre of Abu Iyad, who was second in command to PLO chief Yasser Arafat and a longtime confidant; and Hael Abdel Hamid, known as Abu Hol, another key PLO operative, who handled security affairs. An adviser to Abu Iyad, Mohammed Fakr Omari, or Abu Mohammed, was also shot dead.

The Palestinian news agency WAFA, which had initially blamed the Iraelis, issued a communique today accusing "an infiltrated agent who had ties with a traitorous party."

Palestinian sources identified the suspect as Hamza Abu Zeid, Abu Iyad's own bodyguard, who had previously been a member of the Fatah Revolutionary Council, an extremist group led by Abu Nidal.

Abu Nidal was expelled from Arafat's Fatah organization, the largest group in the PLO, in 1974 after being accused of a series of inter-Palestinian killings. He heads Washington's list of most-wanted terrorists.

One Western diplomat said Abu Nidal had tried to kill Abu Iyad four times previously. He and other diplomats said if the assassinations were carried out by Abu Nidal's group, they were most likely done with the knowledge of Iraqi President Saddam Hussein. In September, Saddam brought some of Abu Nidal's operatives, who had been based in recent years in Syria and Libya, back to Baghdad.

Although Arafat has sympathized with Saddam in the Persian Gulf crisis, Abu Iyad was known to have been suspicious of Saddam and his embracing of the Palestinian cause to justify the Aug. 2 Iraqi invasion of Kuwait. When interviewed in Tunis last June by Washington Post correspondent Jonathan C. Randal, Abu Iyad acknowledged that he distrusted Saddam's ambitions for Middle East leadership, and he emphasized that while Abu Nidal's organization might be small, it was capable of causing havoc.

Senior Israeli officials denied any involvement in the slayings in Tunis, Post correspondent Jackson Diehl reported from Jerusalem. "Israel is not involved," said Defense Minister Moshe Arens in an appearance on Israeli television. "I understand that the assassin was caught and it seems clear that he belongs to Abu Nidal's group, one of the factions of the PLO."

Following the Tunis killings, Israeli soldiers killed two Palestinians and wounded more than 40 during demonstrations today in the Israeli-occupied West Bank and Gaza Strip, authorities said. The Israeli army placed much of the Gaza Strip and several large West Bank towns under curfew to prevent unrest, but Palestinians nevertheless raised black flags in honor of the fallen PLO leaders and gathered in several areas to challenge the army. The curfew was later expanded to cover all of the occupied territories.

Here in Tunis, the families of the three assassinated men held a reception tonight that was attended by Arafat, who broke down in sobs as he offered his condolences, the Associated Press reported. The PLO vowed to avenge the deaths.

PLO officials and diplomats said Abu Iyad, Abu Hol and Abu Mohammed had gathered Monday night to discuss the Persian Gulf crisis. At about 11 p.m., Abu Zeid arrived with a message for Abu Iyad, handed it to him and turned to leave the room. At the threshold, he spun around and sprayed the room with automatic gunfire, killing all three.

Diplomats quoted Palestinian witnesses as saying Abu Iyad's chest was ripped apart by the hail of bullets. The noise alerted neighbors and Tunisian police, who rushed to the villa.

Hoping to escape alive, Abu Zeid then grabbed Abu Hol's wife and daughter as hostages and demanded that a plane be prepared to take him out of the country. Tunisian police stormed the villa, taking Abu Zeid prisoner. The hostages were unharmed.

"I was only five minutes away and I heard the firing of shots," said Khaled Hassan, the head of the Palestine National Council Foreign Relations Committee, in a telephone interview with Post correspondent Nora Boustany in Amman, Jordan.

Abu Zeid had posed as a defector from the Abu Nidal organization and had come to Abu Iyad seven months ago begging for a job, Palestinian sources said.

Palestinian representatives in Amman say Abu Nidal's group is infiltrated by agents of the Mossad, Israel's intelligence service, but the PLO made no official accusations against Israel today. Several sources here in Tunis said they doubted it was an Israeli operation because the killings were done by an insider, and because the Mossad's mode of operations is usually different.

In addition, several independent analysts in Jerusalem told Diehl they speculated the killings were ordered by Abu Nidal or even by one of the Arab states aligned against Iraq, such as Syria.

"In terms of technique, this is not like any previous Israeli operation," said Yossi Melman, author of a recent book on the Mossad.

"Israel has never used Arab hired assasins," he said. "And in all its operations like this, failed or successful, Israel always followed a very strict procedure so that those who pulled the trigger would get away, rather than be caught and interrogated."

Benyamin Ben-Eliezer, member of the Knesset Defense and Foreign Affairs Committee from the opposition Labor Party, said: "Abu Iyad as Arafat's deputy was not exactly enthusiastic about Arafat's total support of Saddam Hussein. So I wouldn't be surprised if this assassination was carried out indirectly by Iraqi forces."

One Arab source said Arafat was en route to Paris Monday night for meetings on a last-minute French initiative to find a peaceful settlement to the gulf crisis when he was informed of the killing. "His plane was in mid-air and he had to turn back," the Arab source disclosed.

Arafat had spent the last few days in Baghdad in a frenzied effort to loosen the language on any kind of linkage between the gulf crisis and the Middle East question but also to pledge support to Iraq in its confrontation with U.S.-led forces.

Abu Iyad had been accused by Israel of founding the Black September underground group, which carried out guerrilla attacks two decades ago, including the 1972 killing of 11 Israeli athletes at the Munich Olympics. He became the chief architect of Arafat's effort to carve a diplomatic rather than a militant role for the PLO on the world stage. Abu Iyad was also Arafat's main ally in promoting dialogue with the United States and was considered the PLO chief's alter-ego.

Balding and a chain-smoker, the heavyset Abu Iyad, 57, had in recent years sought to improve ties with the West by sharing intelligence on major terrorist groups and planned operations.

"One and a half months ago, Abu Iyad boasted to his inner circle that he had exposed 310 such operations against Western interests to French and European counterparts" in the intelligence community, Dr. Asaad Abdel Rahman, a member of the Palestine National Council, said in an interview in Amman today.