Kittipong Kittikachorn, general manager of Suvarnabhumi Airport, said on Tuesday that seven people were critically injured.

"On behalf of Singapore Airlines, I would like to express my deepest condolences to the family and loved ones of the deceased," Singapore Airlines CEO Goh Choon Phong said in a video message.

Officers from Singapore's Transport Safety Investigation Bureau arrived in Bangkok on Tuesday night, Singapore's Transport Minister Chee Hong Tat said.

As the incident involves a US company, Boeing, which makes the 777-300ER aircraft, the US National Transportation Safety Board would be sending an accredited representative and four technical advisers to support the investigation, he said.

The plane encountered sudden extreme turbulence, Goh said, and the pilot then declared a medical emergency and diverted to Bangkok.

Aircraft tracking provider Flightradar 24 said the flight encountered "a rapid change in vertical rate, consistent with a sudden turbulence event", based on flight tracking data.

Agencies via Xinhua