Public threats by Islamic militants holding at least four Americans and two Frenchmen hostage in Beirut have complicated delicate secret negotiations under way through Arab intermediaries seeking their release, according to informed Arab sources.

A plan involving the release in stages of the western hostages and 17 convicted Moslem terrorists held in Kuwait was under discussion when the kidnapers in Beirut suddenly made public their demands earlier this week, making it extremely difficult for either side to show any flexibility now, according to these sources.

The United States was not involved directly in the negotiations but reportedly was aware of the efforts being made to secure the American hostages' release.

Yesterday, a man claiming to speak for the Islamic Jihad, the Lebanese Moslem extremist front, warned in a call to a western news agency in Beirut that it will launch its "greatest military operation" ever against the United States and retaliate against Kuwaiti diplomats worldwide unless the 17 convicted terrorists involved in a spate of bombings in Kuwait in December 1983 are released.

"The U.S. government can expect the greatest military operation that has ever been known," the caller said. "Kuwait's refusal of these demands would mean hell for its counselors and ambassadors the world over."

Earlier in the week, Islamic Jihad released the pictures of six hostages -- four Americans and two Frenchmen -- which were published in the Beirut press together with a warning of "catastrophic consequences" if the group's demands were not met.

The sources said the Islamic extremists' decision to publicize their demands now could reflect a deadlock in the secret negotiations and a desire by the kidnapers to bring more public pressure to bear on both Kuwait and the United States in hopes of getting them to act.

The sources confirmed that the United States has been working for many months through several Arab channels to gain the freedom of the Americans kidnaped in the streets of Beirut and expressed concern that the threats and present publicity over their plight would jeopardize chances for reaching a solution through quiet diplomacy.

The negotiations, according to one source, had been extremely difficult because there is more than one group involved, and some of the kidnapers have family links to the terrorists held in Kuwait.

The four U.S. nationals are Terry Anderson, Associated Press bureau chief; William Buckley, a U.S. Embassy official; the Rev. Lawrence Jenco, head of Catholic Relief Services in Lebanon, and the Rev. Benjamin Weir, a Presbyterian minister. The fate of a fifth American, Peter Kilburn, a librarian at the American Univeristy of Beirut, is unclear.

Despite Kuwaiti and U.S. denials of any willingness to negotiate over the release of the terrorists in Kuwait, Arab sources said that a proposal had been under discussion to get the kidnapers to agree to the release of the hostages against a pledge from Kuwait that it would subsequently free, after a period of time, the 17 convicted terrorists.

The Kuwaitis had reportedly been considering such a way out of the dilemma before another group of terrorists hijacked a Kuwait airlines passenger jet last December and forced it to land in Tehran. The hijackers killed two Americans working for the Agency for International Development and beat several other Americans and Kuwaiti passengers before Iranian security forces put an end to the hijacking.

The hijackers also were demanding the release of the 17 convicted terrorists, but Kuwaiti officials decided to stand fast.

Against the backdrop of the threats to the hostages, the influential Lebanese Shiite religious leader, Mohammed Hussein Fadlallah, yesterday came out publicly against the kidnaping of foreigners in Lebanon. In a statement to Beirut newspapers, he said, "I condemn all kinds of kidnapings by any group."

Fadlallah, who has been implicated by western intelligence services in a number of terrorist incidents, specifically mentioned newsmen and diplomats and called upon his followers to help protect them.

Both the United States and Kuwait reiterated yesterday their longstanding policies of refusing to give into the terrorist demands. In addition, the Kuwaiti ambassador to the United States, Saud Nasir Sabah, said his government did not favor a visit by civil rights activist Jesse Jackson to Kuwait to discuss the hostage crisis.

"It would be counterproductive at this stage," he said. He also said that the issue of Kuwait freeing the 17 convicted terrorists was "not negotiable."

Thursday, civil rights activist Jackson offered to go to Lebanon or Kuwait "if there is any reasonable chance to have an impact." Yesterday he met with the Kuwaiti ambassador on the hostage issue.

President Reagan, asked about a possible effort by Jackson, said, "If Jesse Jackson can do anything, that will be just fine."

Jackson was instrumental in gaining the release in early January 1984 of U.S. Navy Lt. Robert O. Goodman Jr., whose plane was shot down over Syrian-controlled territory in Lebanon.

State Department spokesman Bernard Kalb said yesterday that the department was prepared to facilitate private efforts to gain the release of the hostages but warned that "those persons undertaking such private initiatives do not speak for the U.S. government."