A Swiss International Air Lines cabin crew member on an aeroplane forced to make an emergency landing due to smoke inside the aircraft has died, the flag carrier said on Monday.

The Airbus A220-300 jet, with 74 passengers and five crew on board, was flying from Bucharest to Zurich on 23 December when it had to land in Graz, Austria after engine problems occurred and smoke filled the cockpit and cabin. The plane landed safely.

"We must report, with the deepest of sorrow and regret, that our young colleague died in the hospital in Graz on Monday," Swiss said in a statement.

Swiss chief executive Jens Fehlinger said the loss had left the airline, a subsidiary of Germany's Lufthansa, in shock and grief.

"We are devastated at our dear colleague's death," he said.

"Our thoughts are with his family, whose pain we cannot imagine. I offer them my heartfelt condolences on behalf of all of us at Swiss."

Swiss said in a statement that "out of respect for the loved ones, we will not provide detailed information about our employee or the cause of death."

After the plane made an emergency landing, one cabin crew member was flown by helicopter to a hospital in Graz and taken into intensive care.

Another cabin crew member was also taken to hospital.

All passengers were evacuated and 12 received medical attention. Swiss on Tuesday said that all passengers who were admitted to hospital had since been able to leave.

Chief operating officer Oliver Buchhofer thanked the local emergency services in Graz who had helped passengers and crew.

"This is the saddest of days for us all," he said.

"Losing our colleague and fellow member of our Swiss team leaves me distraught and dismayed."

He said Swiss would work "with the relevant authorities, to determine the causes involved."

Swiss said the focus of its investigation was on the mechanical parts of the aircraft, such as the engine, but also on the use of protective breathing equipment for the cabin crew.