Five American hostages from a commandeered Trans World Airlines flight declared at a press conference here tonight that they feared a military rescue attempt and begged the United States not to try one.

Brought to Beirut International Airport, they said they were well and did not believe their Shiite captors wanted to harm them.

Allyn Conwell, 39, who acted as spokesman for the group, said he visited all 36 of his fellow captive passengers and was assured that the three crew members also were fine. This was the first real confirmation here that there were 40 hostages.

"We want to beseech President Reagan and our fellow Americans to refrain from any form of military or violent means as an attempt, no matter how noble or heroic, to secure our freedom," he said, reading from a handwritten statement.

An oilfield equipment executive from Houston who works in Muscat, Oman, Conwell warned that a "rescue operation would only cause, in our estimation, additional, unneeded and unwanted deaths among innocent people."

Conwell said he was elected by the largest of the scattered hostage groups, and that all had adequate food, water and shelter. He refused to elaborate on how and where they were kept.

Four of the five, who seemed more at ease with the militia representatives whispering in their ears than with the throngs of aggressive reporters, appeared calm and healthy. But hostage Arthur Toga, 33, who was unshaven and nervous, said he wanted to "go home real bad."

A neurologist from St. Louis, Toga looked around the room crowded with journalists and declared that he had been at "the wrong place at the wrong time." He added that he longed to be with his wife, who is seven months pregnant.

The United States has asked the International Committee of the Red Cross to approach Israel to inquire about the status of the Shiite prisoners but has not made a direct request to Israel to arrange a trade. Both nations have stuck to their positions that they will not negotiate with terrorists.

Speaking in a strong, clear voice, Conwell said: "We understand that Israel is holding as hostages a number of Lebanese people who undoubtedly have as equal and as strong a desire to go home as we do.

"The Israeli government reportedly has a plan for releasing these people in conjunction with the military withdrawal of Israel from Lebanon. We sincerely ask and pray that this task be expeditiously completed, especially now that the Israeli forces are south of Lebanon, or almost out of this country."

Pressing his appeal for the release of the Lebanese Shiites held prisoner in Israel, he added: "I feel that most people in America would like to see anyone, anyone in the world unjustly held, returned to their homeland."

Conwell refused to discuss what had happened on the plane or the identity of the hijackers -- except to express relief at the "abrupt change in attitude" when the group now holding the hostages took charge. He described these captors as a much more "amenable, much less hostile, and sincere and fair group of people than we encountered on the plane."

Conwell said his family was now in Corfu, Greece -- "I send them my love, my affection, and hope to God that they are strong and they can wait for me to get back home. I'm ready."

The five hostages also included Thomas Cullins, 42, a Burlington, Vt., architect who said international business forces him to travel a great deal, Peter Hill, 57, of Hoffman Estates, Ill., and Vicente Garza, 53, of Laredo Tex.

Cullens said he thought the prisoners in Israel ought to be released because "We are hostages and they are hostages."

The men were asked how they felt about the refusal of the U.S. government to pressure Israel into freeing the Lebanese prisoners.

The hostages said that they did not have enough information to respond to such questions. They said the hostages had access to only one source of news, which they did not name.

Conwell said the hostages understood that Israel was holding "a number of people hostage" and urged it to hurry arrangements for their release. He added: "It is also our hope, now that we are pawns in this tense game of nerves, that the governments and peoples involved in this negotiation will allow justice and compassion to guide their way."

The news conference took place in a room prepared as if for a tea party. There were trays of raisin cake, tarte alcazar and bottled water on a long table covered with a white cloth in the airport transit lounge.

Angry militiamen halted the press conference after five minutes and shoved the five hostages out of the lounge when an overeager press corps climbed on tables and crowded the hostages and the well-dressed Amal representatives sitting with them.

One BBC correspondent was roughed up and his tape recorder confiscated following a shouting match with an irate Amal fighter with a Kalashnikov assault rifle.