Chicago: A daughter of the Kentucky physician at the centre of a global uproar over his forced removal from a United Airlines flight said Thursday the family was "horrified, shocked and sickened" by the incident.

"What happened to my dad should never have happened to any human being, regardless of the circumstances," Crystal Pepper said at a news conference in Chicago.

David Dao, 69, suffered a concussion, broken nose and damaged sinuses and lost two front teeth when he was pulled from his seat and dragged off a flight Sunday, lawyer Thomas Demetrio said. He said Dr Dao has been released from the hospital and is staying in a "secure" location.

A lawsuit will be filed at some point, and a hearing on preserving evidence is set for Monday in Chicago, Mr Demetrio said. 

"For a long time airlines, United in particular, have bullied us," he told the news conference in Chicago.

"Will there be a lawsuit? Yeah, probably."

He said the law stated that passengers could not be ejected from planes with unreasonable force.

Video of Sunday's incident taken by other passengers and showing Dr Dao being dragged up the plane aisle and with a bloodied mouth circulated rapidly, causing public outrage that was not calmed by the airline's initial response to the case.

Dr Dao's daughter described him as a "wonderful father" and "loving grandfather" who had been returning from vacation in California.

Mr Demetrio said Dr Dao had told him that being dragged down the plane aisle was more terrifying than his experience fleeing Vietnam in the 1970s.

Mr Demetrio and a second attorney, Stephen Golan, said neither they nor the family had heard from United yet.

The lawyers filed an emergency request with an Illinois state court on Wednesday to require United Continental Holdings Inc and the City of Chicago to preserve video recordings and other evidence related to Sunday's incident, which would be a precursor to a lawsuit.

Officials from United and the city could not immediately be reached for comment on Thursday's news conference.

United Chief Executive Oscar Munoz is under pressure to contain a torrent of bad publicity and calls for boycotts against United, including in China, where people have been angered because Dr Dao was an Asian-American passenger.

United shares have lost about 1 per cent of their value since Monday. The stock was down about 0.5 per cent after the news conference on Thursday.

Mr Munoz has sought in the last two days to make amends. In a statement on Tuesday he said he "deeply" apologised and was disturbed by what had happened. On Wednesday, he apologised to Dr Dao, his family and United customers in an ABC News interview, saying the company would no longer use law enforcement officers to remove passengers from overbooked flights.

Mr Demetrio called the apology "staged" and a response to the airline executive's earlier comments, which were heavily criticised by many.

In a letter to employees on Monday, Mr Munoz did not apologise to Dr Dao and defended the airline's actions, saying Dr Dao had been "disruptive and belligerent" in refusing to leave the plane when he was asked to by crew, leaving the staff with no choice but to call aviation security officers for help removing him.

The incident occurred when the airline was seeking to free up space on the plane for crew members it wanted to fly to Louisville.

Chicago city, which Demetrio said had also not contacted the attorneys and family, is also potentially involved in any suit as the airport police who removed Dr Dao from the plane are employed by the city.

Chicago's Aviation Department said on Wednesday that two more officers had been placed on leave in connection with the incident. One officer was placed on leave on Tuesday.

Given the wide public outrage over the incident, Dr Dao is in a strong position as he prepares to launch a legal action, lawyers who represent airlines and passengers said.

"United is looking at a legal claim, but they're also looking at a huge public relations and business problem," said Justin Green, a partner at the law firm Kreindler & Kreindler in New York who represents airline passengers.

"I think United, if they're smart, will quickly and quietly settle the case."

Paul Callan, a civil and criminal trial lawyer in New York, said Dr Dao has at least two potential claims against the airline: a personal injury claim for assault and battery, which could also target the police; and a contract claim.

Mr Callan said he had reviewed United's contract of carriage, the fine print that passengers agree to when they buy tickets. He said that while the contract allows United to deny passengers boarding, it says nothing about removing a passenger from a plane unless the passenger is disruptive.

McClatchy, ​Reuters