A man has allegedly confessed to strangling his childhood friend and watching her die over half-an-hour.

Liam McAtasney admitted to a friend that he killed 19-year-old Sarah Stern who has been missing for months from her New Jersey home, prosecutors said.

A friend had secretly recorded their conversation, they added. He is allegedly heard saying he strangled Stern with such force her body was lifted off the ground, before timing how long it took her to die.

Along with an accomplice, Preston Taylor, he then dumped her body off a bridge after hiding it in bushes for several hours, they added, although the teenager victim's body has never been found.

He is also accused of robbing Ms Stern of thousands of dollars.

Assistant Monmouth County prosecutor Meghan Doyle told the Superior Court in Freehold, New Jersey that he had planned the murder.

"For the two months after that, this defendant spent the entire time calculating every one of his moves to continue to cover up the murder and death of Sarah Stern", she said.

Mr McAtasney's lawyer, Charles Moriarty has maintained his client was innocent.