8-Year-Old Dies After Medical Emergency on Flight to Chicago

10:23 AM PT -- A rep for SkyWest tells us ... "SkyWest flight 5121, operating as United Express from Joplin, MO to Chicago O’Hare diverted to Peoria, IL Thursday morning and was met by paramedics to attend to a passenger in medical distress. We appreciate the efforts of our crewmembers who responded quickly to assist and the medical personnel who met the aircraft."

A child died Thursday after suffering a medical emergency aboard a flight ... despite the crew's best efforts to resuscitate her.

The 8-year-old -- who's been ID'd as Sydney Weston -- was traveling with her family from Joplin, MO to Chicago via SkyWest Airlines ... and reports say she suddenly became ill while on the plane, and then went fully unresponsive about 45 minutes into the journey.

The medical emergency forced pilots to make a detour to General Wayne A. Downing Peoria International Airport ... where first responders met the family at the gate and began life-saving measures.

According to the coroner, Weston did not have a pulse when the plane landed. She was transported to a local hospital where she was later pronounced deceased.

At this point, a cause of death isn't known -- but an autopsy is scheduled for Friday to determine how exactly she may have died.

SkyWest is a regional airline that operates and maintains aircraft used by other major companies like Delta Air Lines, American Airlines, Alaska Airlines and United Airlines. This particular flight was a partnership with United.

We've reached out to SkyWest ... so far, no word back.