Federal Bureau of Investigation spokesman Brett Carr told The New York Times that the agency was sending officials to the scene, but authorities did not believe foul play was involved.
Federal Aviation Administration officials contacted the Marines when the aircraft disappeared from air traffic control radar over MS, officials said.
A Marine Corps spokeswoman said the KC-130 Hercules transport aircraft - which was used as a refuelling tanker - had experienced a mishap.
Banks said most of the dead were Marines. They added that first-responders and emergency workers were wary of ammunition on the plane, according to a Mississippi State Trooper who spoke to WMC Action News 5.
The aircraft crashed in Leflore County just off Highway 82 on Monday evening and according to Leflore Co.
Witnesses described hearing a loud "crackling" noise around the time of the plane crash.
The KC-130 spiraled into a soybean field about 4 p.m. Monday, spreading debris for miles and resulting in fiery wreckage that burned for hours. "Melania and I send our deepest condolences to all!" he wrote. "Further information will be released as available", the Marines posted to Twitter.
She said the plane crashed along County Road 547, which connects acres of farmland between the towns of Itta Bena and Moorhead.
Debris has been scattered in five-mile radius and the search for additional victims is ongoing. Photos and videos showed thick plumes of black smoke billowing from the scene.
The KC-130 aircraft is used for air-to-air refueling, to carry cargo and perform tactical passenger missions.
Firefighters have told local media they had been forced back from the wreckage on several occasions due to "high intensity" explosions.
"The identities of the service members whose lives were lost in this tragic accident are being withheld to allow time for their loved ones to be notified", the Marine Corps said in a statement early Tuesday.