Sydney Weston, described as a "beautiful little light," was flying on SkyWest Flight 5121 when she was killed by an unknown "medical emergency."

The commercial flight made an emergency landing but the young child was confirmed to have died when the flight was grounded.

Little Sydney received "aggressive resuscitative efforts" after falling sick mid-flight, according to officials.

Sydney, from Carl Junction, Missouri, was traveling with her parents Whitney and Dan, and her older brother Evan, with her maternal grandmother Teri Carlson posted a tribute to her granddaughter on Facebook.

The post prompted others to leave their thoughts and prayers for the eight-year-old.

One poster said: "She was such a treasure… it just not real yet.

"I don't know that it ever will be. My Love and hugs are with you all."

Another user added: "What a beautiful little light.

"My heart is so broken for you and her family."

The flight was operated in partnership with United Airlines, which departed from Joplin, Missouri, on Thursday at 6 am.

The plane was headed to Chicago when it diverted to Peoria, Illinois, about 45 minutes after take off.

The girl became ill and then unresponsive before the flight made an emergency landing.