One US National Guard soldier at Mississippi's Camp Shelby died on Friday and another has since been sent to the hospital after both reported symptoms associated with heat injury amid sweltering regional temperatures, the military agency said.

The deceased soldier experienced a medical emergency on Friday after completing a two-mile run as part of a fitness test, the Mississippi National Guard said in a statement. Though the official cause of death is still unavailable, a spokesperson told CNN that on-site medics "reported symptoms associated with heat-related injuries" for both the dead soldier and the one injured on Saturday.

The soldier was immediately assessed by an on site medic and then transported to Forrest General Hospital, where he was pronounced dead shortly after arrival, spokesperson Deidre D. Smith said in a statement.

The National Guard did not identify the soldier but said he was a member of the Illinois National Guard completing a training at Camp Shelby.

The area around Camp Shelby was experiencing triple-digit temperatures on the days the soldiers' symptoms were reported. The nearby southern Mississippi city of Hattiesburg reached record-setting temperatures of 104 and 105 degrees on Friday and Saturday, respectively. The heat index reached at least 110 degrees both afternoons.

The second soldier is receiving care at Forrest General Hospital after experiencing "symptoms of a heat injury" during a training exercise on Saturday, according to the statement.

"Two onsite cadre immediately assisted the Soldier to a shaded area and ensured he was drinking enough water. After a few minutes, the Soldier's condition seemed to worsen" and he was taken to the hospital, the agency said.

The soldier was still receiving care at the hospital on Monday, the statement said.

Shortly after the soldier's injury on Saturday, the Mississippi National Guard published a Safety Stand-Down order which directed all of its members to "cease all outdoor organizational and individual physical fitness training" between 7 a.m. and 8 p.m. until further notice, the statement said.

The entirety of Mississippi was under excessive heat alerts Monday as exceptionally high temperatures hover over the South.

"Safety and readiness are paramount for the men and women serving in the MSNG," the agency said in its statement.

"Leaders are consistently evaluating weather conditions and performing risk assessments to measure and prevent further heat injuries to safeguard our service members and insure we are always ready and always there for the Mississippi communities in which we serve," it added.