The chief executive of United Airlines has apologised for an incident in which a passenger was dragged off a plane, and promised a thorough review of the airline's practices.

The apology came after a torrent of criticism of the carrier's action on a flight on Sunday and its initial explanation of it.

In images now seen around the world, a passenger was forcefully removed and bloodied in the process - the entire event captured on video by passengers and posted on social media.

The 69-year-old passenger, Dr David Dao, had refused to be "bumped" from the overbooked flight - an airline practice that has come under increased scrutiny since the incident.

"I continue to be disturbed by what happened on this flight and I deeply apologise to the customer forcibly removed and to all the customers aboard," chief executive Oscar Munoz said.

"I want you to know that we take full responsibility and we will work to make it right."

The comments were in stark contrast to the company's initial response, in which it seemed to at least partially blame the passenger.

US media published an email Munoz sent earlier to employees, in which he said the passenger "defied" authorities and "compounded" the incident.

"Our employees followed established procedures for dealing with situations like this," he wrote.

Racism allegations

Munoz said on Tuesday that the company will conduct a "thorough review" of its procedures, including "how we handle oversold situations" and how the airline partners with airport authorities and law enforcement.

But the public relations damage was done, with calls for boycotts and the US Department of Transportation promising a review of the airline's actions.

The fact that Dr Dao, an American citizen, was of Asian origin has raised questions of racism, not least in China, a major destination for United.

Calls for a boycott of the airline have appeared on Weibo, China's version of Twitter.

"United Airlines forced passenger off the plane" quickly became the most talked about topic with 580 million posts.

A lot of Weibo users said they felt such an incident happened because the victim happened to be an Asian.

In the US, the head of the Chinese American Citizens' Alliance, an advocacy group, said he may ask members not to fly United to the group's upcoming convention in Chicago.

An attorney for Dao's family said he is in a Chicago hospital getting treated for his injuries.

"The family of Dr Dao wants the world to know that they are very appreciative of the outpouring of prayers, concern and support they have received. Currently, they are focused only on Dr Dao's medical care and treatment," said Chicago lawyer Stephen Golan.

It was the second time in about two weeks that the airline found itself in the middle of a firestorm.

In late March, two teenagers were prevented from boarding a flight in Denver because they were wearing leggings.

The airline defended its action at the time by saying the girls were flying on passes that required them to abide by a dress code in return for free or discounted travel for family members of staff.

Overbooking of flights

The passenger on the overbooked Sunday plane en route from Chicago to Louisville, Kentucky, was one of four people involuntarily bumped to make room for a United flight crew.

The incident shone a light on the practice of overbooking and bumping passengers off flights, which airlines increasingly rely upon to avoid losing money on empty seats when some passengers do not show up for scheduled flights.

If they were to stop overbooking, "the only way of trying to compensate for that over the long term would be to raise fares on everyone else", industry analyst Robert Mann told the AFP news agency.

Instead, airlines sell more tickets than there are seats on a plane, and are generally able to properly forecast demand to avoid major disruptions in getting passengers to their destinations, Mann said.

But they sometimes miscalculate and inadvertently book more passengers than a flight can handle.



In those instances, airlines offer travel vouchers and cash compensation to entice passengers to voluntarily give up their seats for later flights.

When such enticements do not work, airlines have wide latitude under the law.

"If you're still in the terminal waiting to board, you can be told you can't board, even if you have a reservation," Mann said.

"And once you're on board, you are subject to being deplaned based on the order of the crew. So you don't really have any rights."

Last year, 434,000 passengers volunteered to be bumped from flights, while another 40,000 were bumped involuntarily and compensated.