One person has died and several others injured when a Singapore Airlines flight from London to Singapore was hit by severe turbulence, the airline says.

The Boeing 777-300ER was diverted to Suvarnabhumi Airport in Bangkok, Thailand, where it made an emergency landing at 3:45pm (08:45 GMT) on Tuesday.

Flight SQ321 “encountered severe turbulence en route”, Singapore Airlines said in a statement on its Facebook page.

“We can confirm that there are injuries and one fatality on board the Boeing 777-300ER,” it said.

Four hours after the emergency landing, 18 people remained hospitalized while another 12 were treated on outpatient basis, read the statement.

“The remaining passengers and crew are being examined and given treatment, where necessary,” it added.

There were 211 passengers and 18 crew members on board.

Kittipong Kittikachorn, general manager of Suvarnabhumi Airport, told a news conference that the victim was a British man who appeared to have suffered a heart attack.

Airports of Thailand group said the lightly injured and non-injured passengers are being assisted at a specially assigned location inside the terminal at the Suvarnabhumi Airport.

After about 11 hours of flying time, the aircraft sharply dropped from an altitude of about 37,000 feet (11,278 metres) to 31,000 feet (9,449 metres) within five minutes as it crossed the Andaman Sea and neared Thailand, according to FlightRadar 24 data.

“Suddenly the aircraft starts tilting up and there was shaking so I started bracing for what was happening, and very suddenly there was a very dramatic drop so everyone seated and not wearing seatbelt was launched immediately into the ceiling,” Dzafran Azmir, a 28-year-old student passenger, told the Reuters news agency.

“Some people hit their heads on the baggage cabins overhead and dented it, they hit the places where lights and masks are and broke straight through it,” Azmir said.

Deaths linked to turbulence ‘rare’

Andrew Charlton, director of Aviation Advocacy, a consulting firm, told Al Jazeera that deaths caused by turbulence are “extremely rare”.

He said the plane was flying over a tropical area where thunderstorms, which can cause turbulence, are common.

Charlton said a change in air temperature, caused by a lower layer of air cooling down as the day ended and the night began, could also have led to severe turbulence.

He emphasised that the Singapore Airlines aircraft is designed to “withstand severe amounts of turbulence”. However, he also warned that turbulence can still cause injuries to passengers not wearing seatbelts.

The airline did not say how many people were injured, but multiple Thai media reports said there were 30 injured.

It said its “priority is to provide all possible assistance to all passengers and crew on board the aircraft” and that it was working with the Thai authorities “to provide the necessary medical assistance”.

Thai immigration police said medical personnel had boarded the plane to assess injuries, but did not confirm the number. It said the uninjured passengers had disembarked.

Singapore’s Transport Minister Chee Hong Tat said the government would provide assistance to the passengers and their families.

“I am deeply saddened to learn about the incident onboard Singapore Airlines flight SQ321 from London Heathrow to Singapore,” he posted in a statement on Facebook. “My deepest condolences to the family of the deceased.”