By Alanna Petroff

LONDON (CNNMoney) -- Last month, United Airlines CEO Oscar Munoz was named U.S. Communicator of the Year by the magazine PRWeek.

Now he's being raked over the coals for his response to a violent passenger incident on a United Airlines flight.

Public relations experts say the CEO should have quickly offered an unreserved apology after a customer was filmed on Sunday being forcibly removed from his seat and dragged down an overbooked aircraft's aisle.

Instead, Munoz apologized only for "having to re-accommodate ... customers."

Many customers found the response to be overly callous -- and said so on social media, where video of the incident had gone viral.

But Munoz doubled down in a letter sent to United employees on Monday afternoon, describing the passenger as "disruptive and belligerent." He also said that "employees followed established procedures for dealing with situations like this."

Rupert Younger, a PR expert and director of the Oxford University Centre for Corporate Reputation, said that Munoz's response was a major disappointment, and that United should have moved much more quickly to limit damage from the video.

"The apology by the CEO was, at best, lukewarm or, at worst, trying to dismiss the incident," said Younger. "The CEO should make a better, more heartfelt, more meaningful and more personal apology."

Younger said that United should also address public concerns over industry policies related to bumping passengers from their flights.

The United response "looks uncaring and it looks like it's effectively trying to apologize for the incident without really addressing the core issue of how they deal with customers," said Younger, co-founder of the PR firm Finsbury.

United is now struggling to contain fallout from the incident. On Tuesday, the top trending topic on Twitter in the U.S. was #NewUnitedAirlinesMottos, with users suggesting slogans such as "not enough seating, prepare for a beating."

Company shares, which weathered the storm on Monday, were sharply lower in premarket trading.

Meanwhile, video of the incident was attracting huge attention in China, which is a key growth market for United.

United was the top trending topic Tuesday on Weibo, China's answer to Twitter, attracting more than 100 million views.

James Fallows, a journalist who has written a book about China's aviation industry, called out United for failing to respond while the incident dominated social media discussions in such a key market.

"[United is] suffering profound damage in most important international market right now, and no sign of response," he said in a post on Twitter.

Ed Zitron, a PR expert and the author of "This Is How You Pitch," said that United may not be offering a full apology because of fears over a potential lawsuit.

But that's not an effective strategy, he said.

"Had United shown compassion and intent to make things right, they could have come out of this at the very least looking like an airline that cares," Zitron said. "Instead they've just made it even worse."