WASHINGTON (AP) _ An FBI agent testified Tuesday that law enforcement officers used reasonable force in arresting and detaining a Lebanese terrorist suspect charged with participating in the June 1985 hijacking of a Jordanian airliner in Beirut.

Special Agent David W. Johnson, head of the FBI's Hostage Rescue Team, said Fawaz Younis was taken easily when he boarded a yacht shortly after noon on Sept. 13, 1987, and was forced to the boat's deck by two other FBI agents.

Johnson's testimony came during a pre-trial hearing before U.S. District Judge Barrington D. Parker on defense motions to supress statements that Younis made to his FBI interrogators.

The defense contends that Younis was ''seasick and bewildered'' and in pain when Younis talked to FBI agents aboard naval vessels before his transfer to the United States. In court papers, the defense also said Younis ''had no background on which to understand his rights nor the significance of the waiver'' of his rights.

Johnson, an 18-year FBI veteran, testified that Younis was treated humanely during his four-day stay aboard the USS Butte, an ordnance supply ship, his transfer to the USS Saratoga, an aircraft carrier, and his 13-hour air trip to Andrews Air Force Base.

Aboard the Butte, Younis was given three meals a day and coffee or tea, permitted to use the rest rooms and allowed to shower at least once a day, Johnson said. Younis also was permitted on the deck for as long as he wanted, although he was kept separated from the ship's crew.

Johnson also said, however, that Younis was sedated before the flight to the United States because of the possibility of his attacking the pilot and co-pilot in the five-man jet and ''trying to crash the aircraft.''

In answers to questions by the prosecution and defense, Johnson said the FBI agents loosened the handcuffs and leg irons when Younis, a former Shiite militiaman from Beirut, complained that they were hurting him.

He said Younis also showed signs of seasickness and attempted to vomit, so an agent put a treated patch on the back of his ear, a proven method of preventing the illness.

Defense attorney Frank D. Carter also asked Parker to suppress the statements by Younis because, he said, Younis was illegally arrested by the FBI, which unlawfully relied on the Navy in making the capture 13 miles off the coast of Cyprus.

''The law will clearly indicate that the U.S. military cannot enforce civilian law...,'' he said. ''This case is unprecedented in history in the involvement by the military during a peacetime operation.''

Assistant U.S. Attorney Karen A. Morrisette said the law only pertains to the Army and Air Force and not the Navy. She also said the law only prohibits ''the military from acting as law enforcement officials.''

''They can't act as cops,'' she said. ''They cannot participate in arrests. They can act as a taxi cab. And this is the case here. The FBI called for a taxi cab to transport their prisoner. At no time did the Navy have control over the defendant.''

The hearing continues Wednesday.