A missing external panel on a Boeing plane was detected when it landed at an airport in the US state of Oregon on Friday, says United Airlines.

A spokesperson said United flight 433, from San Francisco, arrived at the Rogue Valley International Airport in Medford, Oregon, at about 11:30 (18:30 GMT) on Friday.

The 25-year-old Boeing 737-800 was carrying 139 passengers and 6 crew.

No-one was injured - the missing panel went unnoticed during the flight.

Boeing is under fierce scrutiny after a series of high-profile safety incidents.

Amber Judd, a senior official at Rogue Valley International Medford Airport, said the plane landed safely and the external panel was only discovered missing during a post-flight inspection.

"We'll conduct a thorough examination of the plane and perform all the needed repairs before it returns to service," she said.

An investigation would be carried out "to better better understand how this damage occurred", she added.

The missing panel was next to the landing gear, on the underside of the aircraft, according to images of the aircraft on social media. All outgoing and incoming flights were paused at the airport to search for debris, none was found.

The US Federal Aviation Authority (FAA) said it was investigating how the panel came apart.

Boeing came under renewed scrutiny after a January incident involving a Boeing 737 Max 9 - part of a new range of planes to succeed older 737s - saw an unused cabin door blow out a few minutes after take-off in January.

The incident onboard Alaska Airlines flight 1282 left a gaping hole in the side of the plane and forced an emergency landing.

A safety board investigator examines the broken panel two days after the incident in January

Testifying before US lawmakers on 7 February after the Alaska Airlines blowout incident, the head of the FAA, Mike Whitaker, said inspections of 737 Max aircraft had shown that "the quality system issues at Boeing were unacceptable and require further scrutiny".

Initial findings of a probe found that four key bolts that were meant to lock the unused door to the fuselage appeared to be missing. Mr Whitaker said that Boeing would be held accountable for any future failure or refusal to comply with the FAA.

Earlier this month, fumes detected in the cabin of an Alaska Airlines Boeing 737-800 bound for Phoenix forced pilots to return to Portland airport.

On Monday, at least 50 people were injured after a Boeing 787-9 Dreamliner flying from Australia to New Zealand suddenly dropped without warning, leaving passengers who were not wearing seatbelts tossed into aisles and flung into the ceiling.

In response to that incident, Boeing has told airlines that pilots need to check their seats - after reports said a flight attendant accidentally hit a switch in the cockpit which pushed the pilot's seat forward into the controls - forcing the plane's nose down.

Despite the reported incidents, commercial aviation remains one of the safest ways to travel, experts and regulators say.

Every passenger's worst nightmare: the terrifying moment a plane door rips away mid-air.