A three-year-old boy has accidentally shot and killed his mother as she changed her younger daughter's nappy, police have said.

Christa Engles, 26, was found with a gunshot wound to the head at her home in Tulsa, Oklahoma, on Monday afternoon.

The toddler is understood to have found the gun on a table near the sofa, Tulsa World has reported.

Police believe the toddler then accidentally fired the gun as his mother changed the nappy of his younger sister.

The woman was discovered when the children's grandmother arrived at the house.

Sergeant Dave Walker, of Tulsa Police Department, said a number of guns were found inside the property.

He told NBC News: "We are prepared to do what we have to do to keep the kids safe, but I think this is a horrible, horrible accident."

The woman's husband, a truck driver, returned home a few hours later to learn of the incident.

In a separate incident in Washington, it was reported on Sunday that a three-year-old boy was in a serious condition in hospital after he was shot in the mouth by his four-year-old neighbour while the two were reportedly playing with a gun.

At the time police said they did not have any information about the gun or where the children got it from.

They said the four-year-old may have accidentally shot the three-year-old, but it was not yet clear who owned the weapon, whether it was left unsecure or not or what had led to the gun being fired.

Authorities said both families were co-operating with investigators.