A Korean Air flight to Taiwan was forced to return to Incheon airport west of Seoul after a sudden depressurization on the plane, a Boeing 737 Max 8, the transport ministry said Tuesday.

The ministry said 19 of the 133 people aboard the flight Saturday were sent to hospitals due to ear pain and nosebleeds, but none suffered serious injuries.

The airline and the ministry said the cause of the problem was under investigation. The aircraft was grounded and the ministry ordered South Korea's 11 airlines to examine pressurization systems in all their 400 aircraft.

The sudden depressurization occurred about 50 minutes after the flight's departure.

Separately, Malaysia Airlines said one of its flights en route to Bangkok on Monday made a U-turn back to Kuala Lumpur after the Airbus A-330 experienced a “pressurization issue.”

Malaysia Airlines said its pilots initiated an emergency descent even though the aircraft had not reached the altitude of 8,000 feet and oxygen masks were not deployed. Flight MH780 was carrying 164 passengers and 12 crew members.

An investigation was underway.

The 737 Max has a troubled history. After Max jets crashed in 2018 in Indonesia and 2019 in Ethiopia, killing 346 people, the FAA and other regulators grounded the aircraft worldwide for more than a year and a half.

Concerns over the company's best-selling commercial aircraft were renewed after a panel blew out of a 737 Max during an Alaska Airlines flight in January. No one was seriously injured in the incident.