A four-year-old US girl shot and killed her four-year-old cousin in what authorities have called a tragic and avoidable accident.

The children were playing at their grandfather’s home in Detroit on Thursday afternoon when the girl found a loaded rifle under a bed.

The boy was shot once in the chest and was pronounced dead in hospital.

Police have said they are investigating whether to press charges. It was not immediately known who owned the gun.

A family friend was the only adult at the home at the time, police said.

“It’s a tragic incident,” Detroit police Sgt Michael Woody said. “It’s really not anything more than that. There was no malicious intent.”

“This is a very senseless… incident that could have been avoided,” he said, adding the accident reinforced the need to keep firearms secure, especially around children.

The gun was a small-calibre rifle, Sgt Woody said.