The man dragged off a United Airlines flight in an incident that sparked an international uproar suffered a broken nose and concussion, his lawyer said, adding that he is planning to sue.

David Dao has been released from the hospital, attorney Thomas Demetrio said at a news conference on Thursday during which a member of Dao's family spoke out for the first time.

Dao's lawyers filed a petition in court requesting that the city, which operates O'Hare International Airport, and United Airlines preserve evidence related to the incident. They also said a lawsuit was forthcoming.

"This lawsuit, among other things, hopefully, will create a not just national discussion, but international discussion, on how we're going to be treated going forward," Demetrio said.

"For a long time, airlines, United in particular, have bullied us."

David Dao, a 69-year-old Vietnamese-American doctor, was hospitalised after Chicago aviation police dragged him from the plane to make space for four crew members on the flight from the city's O'Hare International Airport to Louisville, Kentucky.

Demetrio said Dao had told him that being dragged down the plane aisle was more terrifying than his experience fleeing Vietnam in 1975.

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

Chicago runs the airport and the city's department of aviation employs the three officers who dragged Dao off the plane.

'Horrified, shocked and sickened'

Dao, who was discharged from the hospital on Wednesday night, suffered a significant concussion, a broken nose and lost two front teeth in the incident, and he will need to undergo reconstructive surgery, Demetrio said.

Video of Sunday's incident taken by other passengers and showing Dao being dragged up the plane aisle and with a bloodied mouth circulated rapidly, causing public outrage.

Dao's daughter, Crystal Dao Pepper, told the news conference that the family was "horrified, shocked and sickened" by what happened to her father.

One of Dao's five children, Pepper, 33, called him a "wonderful father" and "loving grandfather" who had been returning from vacation in California.

"What happened to my dad should have never happened to any human being," she said.

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

United, in a statement, said CEO Oscar Munoz and the company "called Dr Dao on numerous occasions to express our heartfelt and deepest apologies." The company did not say how it would respond to any litigation, or whether the airline would try to settle.

Dao's 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 the incident, which would be a precursor to a lawsuit.

At a later city council aviation committee meeting, Chicago Department of Aviation Commissioner Ginger Evans said the department was investigating the incident and reviewing its training.

United Vice President John Slater said he was not at liberty to say who called the aviation police, but ruled out the plane's captain. United has no set policy for physically forcing passengers to deboard, he added.

CEO Munoz is under pressure to contain a torrent of bad publicity and calls to boycott United, including from China, where people have been angered because Dao was an Asian-American passenger.

Demetrio said he does not believe Dao's race played a role in what happened.