United Airlines Chief Executive Officer Oscar Munoz has issued an apology for the treatment of a passenger dragged from aboard one of its flights before take off on Sunday.

"I want you to know that we take full responsibility and we will work to make it right," he said in a statement, reversing earlier decisions not to apologise directly for the incident.

It comes as United Continental Holdings Inc shares fell as much as 4.4% after a worldwide backlash erupted over the incident.

Video showing a man who appeared to be Asian being snatched from his seat, his limp body pulled from the passenger cabin of United Airlines Flight 3411, sparked an outcry when the footage went viral.

On Chinese social media, the incident attracted the attention of more than 340 million users on the Weibo platform.

United Continental got about 14% of its 2016 revenue from flying Pacific routes.

In a letter circulated to employees yesterday and seen by Reuters, Mr Munoz declined to apologise for the rough manner in which the passenger was handled, instead expressing regret that "this situation arose" and telling staff that "there are lessons we can learn from this experience."

"While I deeply regret this situation arose, I also emphatically stand behind all of you, and I want to commend you for continuing to go above and beyond to ensure we fly right."

Much of the adverse response centred on whether the man would have faced the same treatment if his ethnic background had been different.

According to Tyler Bridges, a passenger who was on board the flight from Chicago to Louisville, Kentucky, the man who was dragged off before takeoff said repeatedly that he was being discriminated against because he was Chinese.

"He said, 'I'm a doctor; I need to see patients," Mr Bridges, a civil engineer from Louisville who recorded much of the incident on his phone, said.

The Chicago Department of Aviation said in a statement that one of the officers did not follow protocol and added that he had been placed on leave pending a review for actions not condoned by the department.

The US Department of Transportation said it was reviewing whether United complied with overbook rules that require airlines to set guidelines on how passengers are denied boarding if they do not volunteer to give up their seats.

"While it is legal for airlines to involuntarily bump passengers from an oversold flight when there are not enough volunteers, it is the airline's responsibility to determine its own fair boarding priorities," a DOT spokesperson said in a statement.

In heavy trading, shares of United Continental pared losses at mid-afternoon to about 3% at $69.42. The decline shaved about $600 million from the company's market value and made the stock among the biggest percentage decliners in the S&P500.

Late yesterday, United Airlines had also reported its March operational results, saying it expected first-quarter consolidated passenger unit revenue to be about flat with a year earlier.

UBS analyst Darryl Genovesi said in a research note that the revenue projection "may be seen as a modest beat relative to investor expectations."

United plans to report quarterly results next week.