Video of a 69-year-old passenger being brutally dragged off an overbooked United Airlines plane by security, has aroused outrage online.

The airline's chief executive officer, Oscar Munoz, apologized for the inconvenience caused by the overbooking, but hasn't expressed regret for the way the airline treated the passenger. One of the security officers involved has been put on leave pending a full investigation.

In a letter written by Munoz to his employees, he stressed that actions of the United Airline staff members were reasonable and followed the correct procedure for dealing with involuntary boarding denial. He also said he would stand behind his employees.

The U.S. Department of Transportation says they are looking into the incident to work out whether the airline complied with the relevant rules.

The plane, due to depart from Chicago O'Hare International Airport to Louisville, Kentucky on Sunday evening (local time) saw overcapacity as four additional United Airline personnel needed to board the same plane.

The airline then sought volunteers to give up their seats and move to a later flight, offering compensation for doing so. As no passengers responded, staff decided to randomly pick four passengers to leave the plane.

The man at the center of the controversy, said he was a doctor and was returning home to see his patients and refused to get off. Afterwards flight crew called security officers who forcibly dragged the man out of the plane.

In a video posted by one of the other passengers, the man is seen being dragged from his seat by security officers and then dragged on his back through the aisle of the plane with blood visible on his face. The man is screaming, while other passengers plead with the officers to stop.

Another video clip shows the victim returning to the airplane later chanting 'Just kill me. Just kill me' and 'I need to go home.'

The videos spread rapidly online and generated heated discussion on social media. Many have expressed their anger toward the airline. One Chinese aviation insider said plane overbooking was very common but handling the case in such a way went too far.

United Airlines say they are trying to contact the victim to resolve the situation.

(CHINA PLUS April 11, 2017)