Passengers aboard an Air Europa flight diverted to northeastern Brazil due to severe turbulence on Monday said they feared for their lives during the incident, which injured around 40 people, four of whom were taken to intensive care units.

"There are passengers with fractures and injuries to their arms, faces and legs," one passenger told Reuters in the Brazilian city of Natal, where flight UX045 from Madrid to the Uruguayan capital, Montevideo, made an unscheduled landing just after 2:30 a.m. local time.

"It was a pretty horrible feeling. We thought we were going to die there," said the man, Stevan, giving only his first name.

Another passenger, Maximiliano, said the plane — a Boeing 787-9 Dreamliner with 325 passengers on board — had suddenly started to plunge.

"From one moment to the next, the plane destabilized and went into a dive," he said.

"The people who didn't have seatbelts went up in the air and hit the ceiling, and they got hurt — those who had seatbelts on, not so much."

4 people in ICU

Air Europa said in a statement on Tuesday that six people were still in hospital in Natal and were being assisted by ground personnel from Brazil and Spain.

"Air Europa deeply regrets what happened, as well as the inconvenience caused to its customers. The airline hopes for a speedy recovery of the affected passengers and is at the disposal of all its customers to assist them," the airline added.

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The secretariat from the government of Rio Grande do Norte state, where Natal is located, said on Tuesday that four people had been taken to intensive care units, from a total of 40 passengers that had been sent to hospitals, up from some 30 initially reported on Monday.

Air Europa said a replacement flight to take the passengers on to Montevideo had left earlier in the day, with passengers transported from Natal to Recife's larger airport by bus.

Passenger on Singapore-bound flight killed in May

It added that the aircraft was being reviewed to establish the extent of the damage.

The airline did not immediately respond to a request for further comment about the incident, which took place less than two months after a passenger died and dozens were hurt when turbulence hit a Singapore Airlines flight from London to Singapore.

Some scientists have warned of worsening turbulence tied to climate change.

The most common cause of turbulence is unstable weather patterns that trigger storms. These can normally be detected by weather radar, allowing pilots to fly around them.

WATCH | Deadly turbulence on Singapore Airlines flight:

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One passenger died and dozens were injured after a Singapore Airlines flight from London hit severe turbulence and was forced to make an emergency landing in Bangkok. Andrew Chang explains what makes this type of turbulence both invisible and dangerous.

Recent incidents have increased concern in the air travel industry about seatbelt and safety practices.

An international agreement, the Montreal Convention, makes airlines liable for physical injuries from accidents on international flights, which can include turbulence, regardless of whether they were negligent.