AN American man accused of endangering the safety of a transatlantic flight chased a flight attendant up the aisle after she refused to serve him cheese and crackers, a court has heard.

The head flight attendant on the United Airlines Boeing 777 claimed Jeremiah Mathis Thede was "agitated" and demanded to know the name of the colleague who had not given him the snacks.

Sheila Wire, the purser in charge of the flight crew, said later in the flight Thede also allegedly threatened a fellow passenger who had told her he had placed a potential "trip hazard" - a discarded food tray - in the path of attendants walking backwards down the aisle.

The airliner, carrying 264 passengers, was en route from Rome to Chicago on June 20 last year when the captain made the decision to touch down at Belfast International Airport after concerns were raised about Thede's behaviour by cabin crew and other passengers.

Californian Thede (42) denies a charge of endangering an aircraft or persons in the aircraft.

Ms Wire, who has 43 years' service with United, was the first witness called as the trial began at Antrim Crown Court.

She told the court how she had to brief four other male passengers to prepare themselves in case Thede had to be physically subdued.

In the event, police officers removed him from the flight when it touched down in Northern Ireland.

Ms Wire described how she was first alerted to a problem in the economy section between 30 and 45 minutes into the flight.

She said she arrived to see attendant Lisa Hall rushing toward her, with Thede following her.

"I just saw this look on her face, she was stunned," said Ms Wire.

Ms Wire said Thede, from Berkeley, had a pad and pen in his hands and was repeatedly saying "I want her name".

After directing Ms Hall to go to the cockpit, Ms Wire said she tried to establish what had happened from Thede.

"He responded that he went to the back, he wanted cheese and crackers, and 'that lady wouldn't give them to me'," she told the judge.

"I stood there waiting for more information but it was just that, that was really the basis of what was happening at that minute.

"I remember standing there taking a very deep breath and I remember in my mind thinking 'this is over cheese and crackers?'."

Thede, dressed in a light grey suit and white shirt, sat in the dock listening as the purser gave evidence to the jury. He has been on bail in Northern Ireland awaiting trial since last June.

Ms Wire claimed that at other points in the flight Thede was getting up and down from his seat and opening and shutting the overhead bins when the seatbelt signs were on, and refused to sit down when warned.

She said his behaviour prompted numerous complaints from other passengers.

"One said 'what is United Airlines doing to ensure my family and I are safe?'," she said.

"I remember that as a direct quote."

Ms Wire said she warned Thede that the flight might have to be diverted if his behaviour continued.

"He looked at me and said 'are you done yet?'," she told the court.

It has been claimed the plane had to dump thousands of litres of fuel before making the unscheduled stop.

As the crew would have exceeded their legal flying hours if the aircraft had resumed the journey straight away, the passengers were forced to wait almost 24 hours before the plane could take off again, with many having to sleep on the terminal floor.

The trial continues.