The CEO of United Airlines' parent company has said he was "upset" to see and hear what happened after video emerged of security officers dragging a passenger off an overbooked United Express flight at Chicago's O'Hare international Airport.

Oscar Munoz defended his employees, saying the passenger was being "disruptive and belligerent."

While Mr Munoz said he was "upset" to see and hear what happened, he said "our employees followed established procedures for dealing with situations like this."

He did not apologise to the man dragged from the plane.

Video showed a man being forcibly dragged along the floor of a United Airlines aircraft by police officers, as one woman shouts: "Oh my God, look at what you did to him!"

United was trying to make room for four employees from a partner airline, meaning four people had to get off the flight to Louisville.

Three got off but the fourth, a man who said he was a doctor and needed to get home to treat patients, refused.

Three men, identified later as city aviation department security officers, then got on the plane.

Two officers tried to reason with the man before a third came aboard and pointed at the man "basically saying, 'Sir, you have to get off the plane,"' said Tyler Bridges, a passenger whose wife, Audra D Bridges, posted a video on Facebook.

One of the security officers could be seen grabbing the screaming man from his window seat, pulling him across the armrest and dragging him down the aisle by his arms.

“Please share this video. We are on this flight,” Ms Bridges wrote. “...This man is a doctor and has to be at the hospital in the morning. He did not want to get off. We are all shaky and so disgusted.”

A passenger was dragged off a United flight after refusing to leave (Jayse D Anspach)

In a letter to the airline's 82,000 employees, Mr Munoz gave a summary of events, which said the passenger "raised his voice and refused to comply with crew member instructions."

He said after becoming "more disruptive and belligerent," security officers "physically removed him from the flight as he continued to resist — running back onto the aircraft in defiance of both our crew and security officials."

Chicago's aviation department said the security officer who grabbed the passenger had been placed on leave.

After a three-hour delay, United Express Flight 3411 took off without the man aboard.

Footage has since been released of the man appearing to say "just kill me" after he was pulled from his seat.