United States police have begun a homicide investigation after the disappearance of a 10-year-old disabled Australian girl who has been living in North Carolina.

Zahra Baker, who has lost a leg to bone cancer and is hearing impaired, moved to the United States with her father when he married American woman Elisa Baker.

The girl was reported missing from the town of Hickory on Saturday and police found a ransom note demanding $US1 million.

Police say no-one outside her immediate family has reported seeing her in weeks, and they have no idea how long she has really been missing.

"The Amber alert for Zahra Baker was cancelled today and the decision to focus this investigation from the missing child, or abducted child, will turn into a homicide investigation," Hickory police chief Tom Adkins told reporters.

"We understand the public wants to help find Zahra and want to form search teams to help locate her.

"The problem is we cannot confirm with any confidence how long Zahra has been missing. Without this information we cannot positively select the area to search for her."

Local journalist Richard Gould says police told the news conference this morning the girl's stepmother had confessed to writing a ransom note.

"She admitted to having written a ransom note that was found on the vehicle's windshield the morning that Zahra went missing, so the stepmother has admitted writing this ransom note," he said.

"Immediately after having made the submission, she demanded a lawyer and police took out a warrant and charged her with obstructing justice."

Zahra's father has broken down on American television, saying he hopes his wife is not involved in her disappearance.

But Adam Baker, who only recently moved to the US after meeting his wife on the internet, has told the Good Morning America program that from the evidence it is possible that Elisa has something to do with it.

"I wouldn't like to think so. On what I've heard so far, it could be possible," he said, but did not elaborate.

Zahra and her father were living in Australia when Adam Baker met his wife-to-be online about two years ago, and eventually moved to the United States, friends said.

Captain Thurman Whisnant of Hickory Police said earlier that Zahra's biological mother was an Australian and had not seen her daughter since she was about eight months old.

- ABC/AAP