A gunman, identified by local media as a prominent border militia leader and a reputed neo-Nazi, shot dead four people including a toddler girl in Arizona on Wednesday before apparently committing suicide, police said.

The gunman opened fire at a house in Gilbert, a suburb of Phoenix, Gilbert Police Sergeant Bill Balafas said. He could not immediately confirm the identity of the gunman.

The Arizona Republic newspaper, citing unnamed police sources, identified the suspected shooter as Jason "JT" Ready, a reputed neo-Nazi who founded the US Border Guard, a militia that hunts for drug traffickers near the Mexican border.

The youngest victim, a girl of between 1 and 2 years old, was still alive at the scene, but later died in a hospital, Balafas said. The other dead were two men and two women. Police believe the shooter was among the dead.

Police found the bodies of two of the adults outside the home and those of the other two adults and the child inside.

Officers recovered two handguns and a shotgun from the scene. However, Balafas said police had been unable to search the house, and confirm the identities of the dead after an unknown liquid was found in two 55-gallon drums outside the house.

"Right now, we have a hold up because we have a potential hazardous chemical situation," Balafas said.

Investigators subsequently found some "munitions" in the house, and called in federal officers to deal with the items, Balafas said.

He declined to comment on whether Ready, who earlier this year set up an exploratory committee to run for the office of sheriff in Arizona's Pinal County, was among the dead.

Balafas said officers were interviewing a witness to the shooting, which investigators believe stemmed from a "domestic situation."

A Facebook page entitled "JT Ready for Sheriff" carried a message from the administrator noting unconfirmed reports that "a cartel assassination squad murdered JT Ready and several of his friends and family this afternoon in Gilbert Arizona."

A Reuters witness saw a black Chevrolet Impala with specialised number plate with the letters "USBG" on it parked outside the home.

A call to the US Border Guard group seeking comment late on Wednesday went unanswered.

Local resident Scott Kirkwood, 43, said neighbours were shocked by the shooting at the home, which he believed was occupied by a family.

"Everyone is shocked and concerned. Nothing like this ever happens in this neighbourhood. It's usually very quiet," he said.

"I heard sirens pulling up in my backyard. When I went to investigate, there was like 30 cop cars, five ambulances, a fire truck, a SWAT team, the whole nine yards - it was just blanketing the neighbourhood," he added.

- Reuters