A man has been sentenced to at least 25 years in prison after being convicted of the 1979 killing of six-year-old Etan Patz in New York City.

It is one of the city's highest profile crimes in history and helped launch a missing children's movement across the United States.

Etan, whose body has never been found, became the first child to appear on the side of milk carton.

Police say Pedro Hernandez confessed to strangling Etan to death as he made his way to school in Manhattan.

Etan's parents, Stanley and Julie Patz, attended the sentencing and told Hernandez they will "never forgive" him for what he did.

"Pedro Hernandez, after all these years we finally know what dark secret you had locked in your heart," Mr Patz said.

"The god you pray to will never forgive you. You are the monster in your nightmares, and you'll join your father in hell."

Defence lawyer Harvey Fishbein said Hernandez was reluctant to stand up in court and speak but had two things he wanted him to say:

He wanted to express deep sympathies for the Patz family, but he also wanted to make sure it was clear he is an innocent man and had nothing to do with Etan's disappearance.

Hernandez was a teenager working at a convenience shop in Etan's neighbourhood when the boy went missing, on the first day he was allowed to walk alone to his school bus stop.

Lawyers have said the 56-year-old is mentally ill and his confession was false, they have vowed to appeal against his conviction.

Etan's case contributed to an era of fear among American families, making anxious parents more protective of children.

His own parents' helped to establish a national missing children hotline and to make it easier for police to share information about such cases.

The May 25 anniversary of his disappearance became National Missing Children's Day.

Stan Patz, the father of the boy, had his son declared legally dead in 2001

Hernandez first became a suspect in 2012 when the Missing Persons Squad received a tip from someone who remembered him speaking of having killed a child in New York.

Hernandez then confessed to police, saying he had lured Etan into the shop's cellar by promising a soft drink, then choked him because "something just took over me".

He said he put Etan, still alive, in a box and left it with kerbside rubbish.

"I'm being honest. I feel bad what I did," Hernandez said in a recorded statement.

His lawyers said he confessed falsely because of a mental illness that makes him confuse reality with imagination. He also has a very low IQ.

Hernandez's February conviction came in a retrial after his first trial ended in a jury deadlock in 2015.