United Airlines CEO Oscar Munoz says he felt "ashamed" watching video of a passenger being dragged off a flight and has promised to review the airline's passenger removal policy.

In an interview with ABC's Good Morning America, Mr Munoz apologised to Dr David Dao of Elizabethtown, Kentucky, his family and the other passengers who witnessed him being forcibly removed from the United Express plane.

Mr Munoz vowed this "will never happen again on a United flight" and that law enforcement will not be involved in future.

Mr Munoz called the embarrassment a "system failure", saying United will review its policies for seeking volunteers to give up their seats when a flight is full.

United was trying to find seats for four employees, meaning four passengers had to leave the aircraft.