A 14-year-old gunman killed at least four people, including two students, and wounded nine more when he opened fire at a high school in the US state of Georgia on Wednesday, law enforcement said.

The shooter – also a student at the school – was taken into custody. He will be charged with murder and tried as an adult, the Georgia Bureau of Investigation said.

Two teachers were also among the dead.

After the latest chapter in America's gun violence crisis – nearly 400 mass shootings this year alone, by one tally – people gathered at a sports field outside Apalachee High School, some forming a circle with their arms linked.

Earlier, school authorities were reported to have sent a message to parents saying they were enforcing a "hard lockdown after reports of gunfire."

"Our school resource officer engaged him," county sheriff Jud Smith told reporters, referring to law enforcement officers employed to work at US schools.

"The shooter quickly realised that if he did not give up that it would end with an OIS – an officer-involved shooting. He gave up, got on the ground, and the deputy took him into custody."

He said police did not yet know if the shooter singled out specific people as targets and authorities had not yet identified the weapon.

After the all-clear was given, parents were invited to the school to be reunited with their children, with long lines of vehicles visible outside.

One student told local media that he saw blood on the floor and a body as he was led out of the building by authorities.

"I heard gunshots go off... I thought it was fake until I heard more gunshots and screaming," said the male student, whose name was not given by the Fox 5 News channel.

The Atlanta Journal-Constitution newspaper quoted 11th grade student Henry van der Walt as texting his mother that he thought "there's a school shooting."

Minutes later, he texted: "I love you."

Local television footage showed ambulances driving across a school field about two hours after the attack was first reported, with scores of vehicles parked around the school.

The shooting occurred near the town of Winder, about 70 kilometres northeast of Atlanta, the state capital.

US President Joe Biden said he was mourning the dead.

"Students across the country are learning how to duck and cover instead of how to read and write. We cannot continue to accept this as normal," he said, referring to the frequency of such attacks across the country.

School shootings have become a sadly regular occurrence in the United States, where about a third of adults own a firearm and regulations on purchasing even powerful military-style rifles are lax.

Polls show a majority of voters favour stricter controls on the use and purchase of firearms, but the powerful gun ownership lobby is opposed to additional restrictions and lawmakers have repeatedly failed to act.

Speaking at a campaign event in New Hampshire after the shooting, Vice President Kamala Harris said it was time to end the "epidemic of gun violence."

Republican US presidential candidate Donald Trump said the perpetrator of the shooting was a "sick and deranged monster."

This year, there have been at least 384 mass shootings – defined as a shooting involving at least four victims, dead or wounded – across the United States, according to the Gun Violence Archive.

At least 11,557 people have been killed in firearms violence this year in the US, according to the GVA. (AFP)