The US Federal Aviation Administration said it is investigating the Sept 15 incident on a flight from Salt Lake City (SLC) in the southwestern state of Utah to Portland, Oregon in the Pacific Northwest.

Delta said in a statement emailed to AFP that the aircraft, carrying 140 passengers, “was unable to pressurise above 10,000 feet”.

“After the plane landed, 10 people were checked or treated by medical personnel who met the flight at the gate,” it added.

“We sincerely apologise to our customers for their experience on flight 1203 on Sept 15,” the statement said.

Delta said the flight crew followed procedures to return to SLC where its team on the ground supported the customers with their immediate needs.

Local station KSL TV interviewed passengers who described seeing people gripping their heads in pain or bleeding from their ears or noses not long after the plane took off.

Passenger Jaci Purser told KSL TV she felt a stabbing pain in her ear.

“I grabbed my ear, and I pulled my hand back, and there was blood on it,” she said.

Delta said the plane’s oxygen masks did not deploy.

“The Boeing 737-900 aircraft was repaired and was back in service the day after the incident,” Delta said.

The plane is not part of the Boeing 737 MAX fleet, which brought renewed scrutiny to the embattled aviation giant in January when a fuselage panel blew out of an Alaska Airlines plane mid-flight, necessitating an emergency landing.