The Syrian officer manning the first of six checkpoints before the entrance to this ancient citadel in the Bekaa flatland curiously stuck his head into our car and asked if my tall Australian companion was "one of the hostages." He was joking.

Shiite residents in this town in the Bekaa Valley dismiss as rumors reports that a group of American passengers taken off TWA Flight 847 after it was hijacked June 14 has been moved from secret hideouts in the Moslem-controlled sector of the Lebanese capital to Baalbek.

But Baalbek is one of the locations considered to be a primary target of possible American retaliation, should any of 40 American hostages be harmed. It is the focal point for many of the most extreme Islamic militant groups operating in Lebanon that have made the United States and Israel their archenemies.

Said one angry Baalbeki, a veterinarian who gave his name as Raad: "America is misbehaving. Why are their planes over Baalbek every night taking pictures?"

Raad, speaking in American-accented English, complained that the Americans were fussing over a few dozen hostages while there were hundreds of Lebanese "rotting in Israeli jails."

The hijackers' main demand for freeing the American passengers is the release of more than 700 Lebanese prisoners held in the Israeli jail of Atlit.

Baalbek is the seat of Hezbollah, or Party of God, whose main spokesman is 32-year-old Sheik Ibrahim Amin and whose spiritual guide is Sheik Mohammed Hussein Fadlallah, the highest religious authority for Shiite Moslems here after Iran's Ayatollah Ruhollah Khomeini. The hijackers of the American airliner are said to be members of the extremist and Iranian-inspired Hezbollah, which is closely allied with Amal Islami, a radical breakaway faction from the mainstream Amal movement of Justice Minister Nabih Berri.

Hezbollah and Amal Islami, headed by Hussein Musawi, have been collaborating closely with Iranian revolutionary guards, also based around here and said to number anywhere from 300 to 900. The Iranians also have set up and funded the Martyr's Association, which supplies money to the poor, pamphlets for strict religious guidance and paint for Hezbollah murals.

Having an outwardly peaceful and rural pace, this Moslem town, 30 miles east of Beirut, can be deceiving. It attracted more than 3 million tourists 10 years ago, matching Lebanon's population.

Colorful paintings depicting bloody battles and anti-American and Israeli slogans break the monotony of the arid countryside in the Baalbek plain.

"Hezbollah are the victors and Hezbollah are everywhere . . . death to America, the great Satan," read one freshly painted red white and blue wall. "I see happiness in death, and life with oppressors as painful," proclaimed a Hezbollah mural in the village of Bouday, 10 miles northeast of Baalbek.

At least a half-dozen Soviet-made Syrian tanks are dug into the fields around Bouday, a tiny village and the home of Sheik Mohammed Yazbeck, who teaches Islamic studies at the ornamented and well-guarded Imam Ali Mosque at the eastern edge of Baalbek. Yazbeck is not a senior figure in Hezbollah's centers of power, but he has a crucial role. He is in charge of inculcating religious beliefs and ardor into the Shiite youths of Baalbek. Barrels and iron rods bar entry into his newly built religious school.

Syrian troops in maroon-and-khaki fatigues are in evidence all over Baalbek and its environs. There are about 30,000 troops from the all-Syrian Arab Deterrent Force stationed in central and northern Lebanon. Syrian military forces have controlled the Bekaa Valley since 1976 when they intervened in the Lebanese civil war.

Syrian soldiers had to step in and disengage Shiite and Palestinian fighters last month when clashes broke out in the Beirut refugee camps.

That pitted Amal against guerrillas in Beirut.

"We are all Hezbollah sympathizers here," said 25-year-old Hussein Shamas, a Lebanese soldier, who comes from Bouday. Faded banners of Khomeini fluttered in the wind throughout his village, dotted with poplar and apricot trees.

"Hezbollah offers the best way. If they want to set up an Islamic republic, I will have no objections," said the soft-spoken young soldier.

He insisted that no one in the region of Baalbek had anything against the mainstream Amal organization of Nabih Berri but that the majority of them felt drawn to Hezbollah because of its "vision and clear-cut aims and openness to all." Lebanon's Sunni Moslem community, by far the more affluent and urbanized, is fearful of the Shiites' growing influence and passion for extremist views.

"Israel should be wiped off the face of this earth," said more Hezbollah-signed graffiti on the outside cement wall of a sharecropper's shed. The one next to it pictured a fist bursting through an American flag.

The hostility to America and Israel is overpowering, especially among guards of Amal Islami, which broke away from Berri in 1982 because he refused to follow Iranian instructions on how to conduct his politics and to boycott a national salvation committee.