At least four people were killed and more than 20, most of them teenagers, injured Saturday night in a shooting at a birthday party in the US state of Alabama , police said, in the latest mass killing in the United States.

According to local reports, the shooting took place at a 16-year-old's birthday party at a discotheque in Didville, northeast of Montgomery, the Alabama state capital.

"Tragically four people were killed in this incident and there are many injuries," Alabama Law Enforcement Agency spokesman Sergeant Jeremy Burkett told reporters Sunday.

Burkett later determined that 28 people had been injured, some critically, and urged anyone with information about the crime to come forward.

Annette Allan said her grandson, Phil Dowdle, was one of the victims. Phil, a senior in high school, attended his sister Alexis' birthday party on Saturday.

"He was a very humble child. He didn't bother anyone. The smile was always on his face," Alan said in an interview with a local newspaper, noting that Dowdel's mother was also injured.

"Everyone is sad," she continued.

President Joe Biden said in a statement that the nation is once again in mourning over the killing of young Americans as a result of gun violence.

Biden, who has long sought to tighten gun restrictions, asked, "How far has our nation come when children can't attend a birthday party without fear?"

He added, "Arms are the main cause of death for children in America, and the numbers are increasing, not declining," stressing, "This is intolerable and unacceptable."

Lake Martin Hospital, close to the site of the shooting, has received 15 gunshot wounds, most of them teenagers, Heidi Smith, an official at Ivy Creek, the company that runs the rural health facility, told AFP.

Smith added that six of the injured were discharged from the hospital and nine were transferred to hospitals with higher capacities, noting that five of them are in critical condition.

More than 12 hours after the tragedy, neither Burkett nor other officials have provided any details about who and what motivated the shooting, or whether the suspect has been arrested.

"We can't share anything else at this time," the sergeant said, adding only that the incident was "related to a birthday party."

Didville Police Chief Jonathan Floyd described the town as "a tight-knit community full of wonderful people."

Alabama police did not give any additional details, but said they had opened an investigation in partnership with the Deadville police and federal agencies, including the FBI.

And the station, "WRP-IL," reported on Sunday about a heavy police movement on Saturday night and cordoning off a building in Didville with crime scene tape, noting that white sheets covered parts of the site.

State leaders took to Twitter Sunday to express their grief, offer prayers for the victims, and denounce the violence, but provided no details of what happened.

Alabama Governor Kay Ivey wrote on Twitter, "This morning, I am as sad as the people of Deadville and the people of Alabama," adding, "Violent crime has no place in our state."

Alabama Sen. Tommy Tubervilla called the shooting "heartbreaking."

The United States, with a population of 330 million, has 400 million firearms, and witnesses frequent bloody mass shootings.

The recent shooting in Didville coincided with the 16th anniversary of the Virginia Tech school shooting in 2007, which is the deadliest in American school history, killing 32 people.

In a separate incident, police confirmed late Saturday that two people were killed and four others wounded in a shooting in Louisville, Kentucky, which last week witnessed a shooting attack at a bank that killed five people.

The Gun Violence Archive has counted 163 mass shootings in the United States so far this year. The non-profit organization defines a mass shooting as one in which at least four people are killed, either injured or dead, excluding the shooter.

Efforts to tighten gun controls are opposed by Republicans who are staunch defenders of the constitutional right to bear arms.

A third of Americans own at least one gun, and nearly half of all adults live in a household with a gun.