Lawyers for a passenger forcibly removed from a United Airlines flight have filed an emergency court request for the airline to preserve evidence.

David Dao was filmed being dragged off the overbooked flight at Chicago O'Hare airport, bloodied and screaming, in a video watched millions of times online.

United Airlines said it would refund the ticket costs of all passengers on Sunday's flight.

The airline's chief executive, Oscar Munoz, is insisting he will not resign.

As of Tuesday, Dr Dao was still recovering in a Chicago hospital, his lawyer said, but a family member is expected to give a news conference on Thursday.

The filing with an Illinois State court demands that United Airlines and the City of Chicago preserve all surveillance videos, cockpit voice recordings, passenger and crew lists related to the flight.

The City of Chicago runs O'Hare International Airport.

The airline has confirmed it is "reaching out" to customers on United Flight 3411 and "offering compensation for their flights".

On Wednesday, Mr Munoz said he felt "shame and embarrassment" and vowed it would never happen again.

Mr Munoz had initially described Dr Dao as "disruptive and belligerent".