U.S. intelligence officials think that a group of passengers from Trans World Airlines Flight 847 is being held captive in a Shiite military barracks in the eastern Lebanese town of Baalbek, an area under Syrian control, government sources disclosed yesterday.

This was one of several developments further diminishing prospects for release soon of the 40 Americans from the hijacked airplane held in Lebanon.

The base in Baalbek is a headquarters of the most radical Shiite elements, who seem least likely to agree to an unconditional release of the 40 Americans. The Baalbek barracks has long been at the top of a list of potential targets for U.S. military retaliation against the militant Shiites -- a fact known to the Shiites, since the barracks has been attacked previously by Israeli and French bombers. So the presence of these Americans could make U.S. military retaliation even more unattractive than it was.

There were conflicting reports about the identities of the Americans reportedly held in Baalbek -- at least six of them, officials said. One possibility, officials said, is that four U.S. military men aboard Flight 847 are among those being kept away from the main group of hostages.

Statements yesterday by Amal leader Nabih Berri and Israeli Foreign Minister Yitzhak Shamir further underscored the increasing difficulty of negotiating the release of the 40 American hostages.

Berri added "one more demand" -- "the withdrawal of the U.S. Sixth Fleet from our coast" -- before Amal would release the hostages, while Shamir reiterated the Israeli demand that there be "quiet" in southern Lebanon before Tel Aviv considers releasing the remaining 735 prisoners, most of them Shiites, in its hands.

Moreover, 12 days into the hijacking and hostage-taking, there are signs that the radical Shiite elements of Hezbollah, the Party of God, who have a headquarters at Baalbek are positioning themselves to dictate the final terms of the hostages' release.

Theoretically, at least, Syria could influence these radicals, since it controls the territory where they operate, but Syrian President Hafez Assad has said publicly that he has been unable to sway them in the past. And Assad could decide that he doesn't want to help the United States out of its hostage crisis.

Berri said Sunday that he has no direct influence over those holding the separate group of hostages.

Even a radical Shiite leader has acknowledged that the situation gets more complex with the passage of time. Sheikh Mohammed Hussein Fadlallah, the influential Beirut Shiite religious leader, warned last week that prolonging the situation could diminish the prospects for a negotiated solution "because the parties keeping the hostages are diversified" and local Lebanese events have a tendency to "become regional and then international."

Gary Sick, the top expert in Jimmy Carter's White House on the Iranian hostage crisis, said in an interview that the Shiite radicals "have much larger objectives in southern Lebanon" and that the longer the crisis drags on "the more Berri has to worry about protecting his flanks and getting something out of Uncle Sam."

Sick and other U.S. analysts say they fear that the radicals will renew some demands the hijackers made initially, such as the end of Israeli support for the Lebanese Christian-led South Lebanon Army (SLA) and the total withdrawal of Israeli soldiers from the "security zone" it has set up just across Israel's border in southern Lebanon.

This fear is based partly on reports that the three original hijackers came from the south and have relatives among the 735 prisoners in Israel who were captured by the Israelis in their crackdown on Shiite opposition in southern Lebanon. This would give those hijackers a special interest in the situation there.

"The radicals may hang on to them for negotiations over southern Lebanon," said one U.S. analyst, referring to the group of hostages held separately.

Early in the hijacking, the hijackers also demanded the release of two Amal members on trial in Spain for allegedly attempting to assassinate a Libyan diplomat there and the release of 17 other Shiites -- convicted terrorists held in a Kuwaiti jail.

(Berri has claimed credit for persuading the hijackers to drop all demands other than the release of the 766 Lebanese prisoners in Israel.)

The United States and friendly third parties have been through a frustrating series of unsuccessful attempts to negotiate a deal with these radical Hezbollah elements to release seven Americans kidnaped in Beirut over the last 15 months.

During those talks, carried out by Arab intermediaries on behalf of the U.S. government, the radicals refused to accept a proposal for the prior release of the Americans against a Kuwaiti commitment to free the 17 Shiite convicted terrorists at an unspecified date.

The radicals insisted that all the prisoners in Kuwait be released first, or at the same time, according to one Arab source.

If the radical Shiite captors of the separately held group of TWA passengers adopt a similarly hard line now, then even the release of a good number of the 735 Lebanese still held in Israel may not be enough to persuade them to release all the 40 Americans in exchange for an Israeli commitment to complete release of their captives after the Americans are free.

This leaves open the possibility that over a period of weeks, the Israelis could continue the process -- begun yesterday -- of releasing Lebanese prisoners in batches of a few dozen until all have been freed. But the continued Israeli release of Lebanese prisoners could begin to look like a unilateral concession to the hijackers -- the one thing American and Israeli officials say they will never consider.

U.S. and Israeli leaders insisted Sunday that there was no "linkage" between the release of the first 31 Lebanese prisoners in Israel and the fate of the 40 American hostages. But eventually, the U.S. and Israel may have to decide how far they are willing to go unilaterally to satisfy the Shiites' principal demand before the hijackers release their American captives.