Outrage spread to Vietnam on Wednesday over United Airlines' handling of a passenger dragged from his seat after it emerged that the 69-year-old U.S. doctor was Vietnamese by birth.

Although United Airlines has no direct flights to Vietnam, there were widespread calls on social media for a boycott after video showed a bloodied David Dao being yanked out of the plane by airport security on Sunday to make way for United employees.

The ire in Vietnam grew quickly after it was reported that Dao's origins were not in China as many had at first assumed.

Vietnamese also fumed at allegations over Dao's past reported in the United States as irrelevant and possibly racist.

"Watching this makes my blood boil, I'll never fly United Airlines," commented Anh Trang Khuya on Facebook, the most widely used social media platform in Vietnam.

Nguyen Khac Huy wrote: "Boycott United!!! This is excessive! Let's be loving and united, Vietnamese people!"

There was no immediate comment from the government or in state media.

Video showing Dao being pulled from United Airlines Flight 3411 at Chicago O'Hare International Airport on Sunday went viral and the worldwide backlash hit the airline's share price and prompted an apology from the company chief executive.

Kentucky's medical board website shows that a doctor David Dao graduated in 1974 in Ho Chi Minh City - then known as Saigon and the capital of U.S.-backed South Vietnam before its defeat and the reunification of Vietnam a year later.

Around that time, Dao left for the United States, according to U.S. media and Vietnamese websites.

Vietnamese media said that Dao was also a songwriter and crooner of soulful ballads - including one about the memory of rain falling in Saigon.

Reports in U.S. media of an offence that had led to Dao losing his medical license in 2003 were dismissed in Vietnam as a probable smear campaign.

"Dr. Dao didn't do anything wrong on that flight and that's the main thing," wrote Clarence Dung Taylor in a post that had more than 4,000 likes.

The attitude to the case shifted dramatically in Vietnam once it was reported that Dao was not from China - with which Vietnam continues to have a maritime dispute over the East Vietnam Sea.

When initial reports had suggested the man being dragged from the plane was Chinese, some Vietnamese had posted strongly unsympathetic comments about him.

"So funny," wrote Bui Nguyen Trong Nghia. "Now they know he's Vietnamese, most people stand up to advocate. Whether it's Vietnamese or Chinese, there'll be discrimination as we're Asian."