Twenty-one people have been killed in a primary school in South Texas in the United States, nineteen of whom are young children while two are adults by a teenage gunman who shot indiscriminately on the premises before he was gunned down by law enforcement.

The 18-year-old shooter identified as Salvador Ramos, armed with a handgun, an AR-15 semi-automatic rifle, and high-capacity magazines, made his way to the Robb Elementary School in the city of Uvalde and opened fire at classrooms at 11.32 am on Tuesday, May 24.

He is believed to have shot his grandmother before heading out for his killing spree and upon arrival, he abandoned his car before entering the school to "horrifically, incomprehensibly" open fire. She however survived the attack.

Texas, Gov. Greg Abbott had initially announced that 14 students and their teacher were gunned down in their classroom but later Tuesday Sgt. Erick Estrada of the Texas Department of Public Safety confirmed the death toll had risen to 19 students and two adults.

The children killed were in the second, third, and fourth grades, and they range in age from seven to 10 years old.

Local media report he may have been a high school student in the area.

Congressman Tony Gonzales, who represents the area, expressed his grief over the shocking incident, calling out the acts of aggression against innocent children.

 "I am heartbroken for our South Texas community. It is devastating when our innocent children become the victims of senseless violence. We are devastated."  he said in a statement.

The mass shooting comes less than two weeks after an alleged white supremacist killed 10 people inside a grocery store in Buffalo. N.Y. Authorities have said the Buffalo shooting was a hate crime in which the suspect targeted Black people.