An eight-year-old boy has been shot and killed by his brother who mistakenly thought the loaded handgun he was holding was a BB gun, US police have said.

Three siblings were visiting their 19-year-old uncle on Saturday when one pulled the trigger of the gun, hitting his brother, named as Sammy Lorenzo, in the chest.

Emergency services were called to the house in Cincinnati, Ohio at around 4pm. The boy was taken to hospital and rushed into surgery, but later died, Cincinnati Police Lt. Don Luck told the Cincinnati Enquirer.

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One of the brothers “kept telling the story of how it happened, over and over again,” Lt. Luck said. “It's so sad.”

“We walked in through the front door here that's in to the kitchen area and the child was laying on his back with a gunshot wound to the chest. He was conscious and alert at that time,” Sgt. Jim Perkins told the Fox 19 news channel.

"I think everyone is pretty much upset, considering the circumstances, and at this point we don’t know where the adult was inside the house and that’s why we’re conducting the investigation at this time," he added.

No charges are expected to be filled against the child, but it's too early to tell whether charges will be filed against any adults, Lt. Luck said.