A six-year-old boy playing with a gun fatally shot his younger brother, 4, in the head, authorities said.

The boys were at their home in Essex County, New Jersey, USA, when the shooting occurred at approximately 11am.

Despite being treated at hospital, the four-year-old boy was pronounced dead at 4.26pm.

The boys' mother, 22-year-old Itiyanah Spruill, has been arrested charged with endangering the welfare of a child and a weapons violation, a police official said.

A spokesman said the gun belonged to the boy's mother. It wasn't immediately known how the 6-year-old got hold of the gun.