Southwest Airlines passengers have expressed concerns that the safety card on their planes suggested that they were on a Boeing 737 Max, which was grounded after the fatal Ethiopian Airlines crash last month.

The airline said that the plane was still grounded and that the safety card is used for more than one plane model, adding that p assenger safety procedures on the 737 Max and the more widely operated 737-800 are identical.

Passengers on at least one other airline have expressed similar confusion.

Boeing 737 Max jets remain grounded around the world as Boeing works on a software update for the aircraft.

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Southwest Airlines passengers have expressed concerns on social media after confusion over onboard safety cards led them to think they were flying on a Boeing 737 Max, the plane that regulators around the world grounded after it was involved in two crashes in five months that killed nearly 350 people.

Some have tweeted at Southwest after noticing that the safety card on their planes said it was for a 737 Max aircraft. One questioned on Sunday whether the airline had actually grounded the plane and described seeing the 737 Max 8 listed on the card as "nerve wracking."

But Southwest said that all its 737 Max jets were grounded following instructions from the Federal Aviation Administration and that it uses the same safety card for multiple plane models, all of which are listed on it.

Read more: Boeing's 737 Max 8 nightmare and troublesome politics threaten the US's standing as the global aviation leader

"Hi, just wondering why I was on a 737 max 8 yesterday," one passenger tweeted at Southwest. "I thought they were grounded until end of May? Nerve wracking!"

Grounded Southwest Airlines Boeing 737 Max aircraft on the tarmac at the Southern California Logistics Airport in Victorville, California. MARK RALSTON/AFP/Getty Images

Southwest replied: "You were not on a MAX 8 aircraft. You were on a Boeing 737 800 series."

It added that the safety procedures for both aircraft models were the same for passengers.

"The Safety Card for Customers is the same card for both the 737 MAX 8 and the 737 800 series," the airline said. "The layout of the aircraft from the Customer perspective is the same."

Southwest has apologized to other passengers for the confusion.

Passengers on at least one other airline have expressed similar confusion. A Norwegian Air passenger tweeted on Wednesday that after takeoff she noticed the plane model was listed on the safety card.

"Boeing 737 max 8!!! Are they not banned?!" she wrote in Swedish. "Panic!"

Norwegian said in a statement on Sunday that its safety cards were designed for both the 737-800 and the 737 Max, The Sun reported.

"The Boeing 737-800 is not affected by the global grounding by aviation authorities of the 737 MAX aircraft," Norwegian said, according to The Sun. "Our safety cards on board display the two model variants since passenger safety instructions on both aircraft are the same."

Debris at the crash site of the Ethiopian Airlines flight. MICHAEL TEWELDE/AFP/Getty Images

Boeing is working on an update to the 737 Max 8's automated anti-stall software system. Last week it acknowledged that issues with the system played a role in the fatal Lion Air crash in October and the Ethiopian Airlines crash in March.

Read more: A timeline of Ethiopian Airlines Flight ET302 shows its pilots fighting desperately to save their doomed Boeing 737 Max jet

The 737 Max fleet will not return to service until the update is approved by the FAA and its equivalent regulators around the world.

Southwest has more exposure to the 737 Max than any other US airline. About 6.3% of its available seat miles were from the 737 Max as of March, Stifel Financial said.

Boeing CEO Dennis Muilenburg apologized on Thursday for the crashes, saying that Boeing was "sorry for the lives lost" and that the "tragedies continue to weigh heavily on our hearts and minds."

Muilenburg added that Boeing had its "top engineers and technical experts working tirelessly" and that the company would give pilots "training and additional educational materials" for the software system.