Family of five shot dead in US

Three children, one as young as four, have been victims in an apparent murder-suicide of a family of five, police in the US state of Maine say.

Three children have been victims in an apparent murder-suicide of a family of five in the US. (AAP)

A family of five, including three children aged from four to 12 years old, were found shot to death inside a US apartment, authorities said.

Investigators said a gun was found near one of the bodies, and it appeared no one outside the family was responsible for the shootings.

Police said a family friend had contacted a worker at the apartment complex worried about the wellbeing of the family.

The worker entered the apartment on Sunday local time and discovered a body, then immediately called police, who later found bodies in three different rooms.

Maine state police Sergeant Chris Harriman said murder-suicide was one of the scenarios being investigated, but that determination would be made by the state medical examiner's office.

Police described the firearm found in the apartment as a long gun. Shell casings were also recovered.

Autopsies are scheduled for Monday.