Syria's chief of military intelligence in Lebanon, the godfather of the four-week-old security plan for Moslem west Beirut, said the gunmen who attacked an American University Hospital bus last week, and killed four people, would be hanged if apprehended.

Brig. Ghazi Kanaan said Syrian troops would use force if necessary to pacify Lebanon's crime-ridden capital, and he offered a reward of $2,300 for information regarding the killers' whereabouts. He said the gunmen would be hanged in the spot where the raid occurred.

Lebanese police have questioned several people during their investigation but have not been able to determine who committed the raid.

"It was a very unfortunate act of sabotage. We will strike at every such act," Kanaan said a few hours after inspecting his troops in west Beirut yesterday. "I hope to hang those who did it in the same place."

"We will show no mercy for those who commit terrorism," Kanaan warned.

On Saturday gunmen killed two doctors, a hospital administrator and a student, and wounded three others, all passengers on a university bus traveling from west Beirut, where the university hospital is located, to the Christian sector in the east of the city. The ambush was the most serious challenge to the security plan.

About 300 elite Syrian soldiers and an undisclosed number of plainclothesmen were dispatched to west Beirut on July 4, for the first time since 1982, to help Lebanese Army troops and police enforce law and order. The plan is being managed by a quasi-government composed of Lebanon's Moslem ministers.

Although contested by Lebanese President Amin Gemayel and other christian leaders, Syrian involvement in the security plan has been welcomed by Moslem leaders frustrated by the state of chaos in Beirut.

Palestinian guerrillas also are opposed to a major Syrian military involvement in Beirut and its suburbs which they see as restricting their movement in Lebanon.

Intensive efforts are underway to extend the Syrian presence and the security plan into Beirut's Shiite Moslem suburbs. The Iranian-backed Hezbollah, or Party of God, which controls and uses the Shiite slums on Beirut's southern edges as a hideout, has already rejected to the idea. It is believed that a number of western hostages kidnaped in Beirut over the past two years are being held by fundamentalist groups in the suburbs. Kanaan indicated that Syria was not eager to get involved in a military confrontation in the densely populated area.

Meanwhile, Lebanese media reports have mentioned the arrival of fresh Syrian troops to Moslem areas in preparation for possible upheaval.