A US sex offender arrested for a gruesome samurai sword murder has made the shocking claim that he has previously murdered six others, also claiming that he is from New Zealand and speaking with a Kiwi accent.

Nicholas Brent Gibson, 32, was arrested by police in New York on May 5 after a manhunt following the brutal murder of a 77-year-old man in Florida.

The body of Erik Stocker was found on April 30 after neighbours reported a foul smell coming from his Miami Beach apartment.

The stockbroker, a former housemate of Gibson, had been stabbed and slashed with a samurai sword.

During questioning by New York police Gibson, who was convicted of rape at the age of 13 in 2000, admitted to the murder and sensationally claimed to have killed six others over the past two decades.

He claimed to police that he killed for the first time in 1999, bashing a man with a brick, going on to murder six others including Stocker and his penultimate victim Essie Puhakka.

The Finnish national's body was found in Florida's Biscayne Bay in February 2015, floating to the surface near a seawall.

Investigators at the time believed she had been in a struggle before she was submerged but could not be sure she drowned.

Police in Florida are now working with authorities across the US to see if Gibson's claims are credible, although doubts have been raised as he was incarcerated at the time some of the crimes were committed.

Among Gibson's claims is that he is a New Zealander, with the convicted sex offender allegedly speaking with a Kiwi accent to lend weight to his assertion.

The Daily Mail spoke to a mental health nurse who had allowed him to stay with her after he was treated in a South Florida mental health unit in 2016.

She states that Gibson claimed he was from New Zealand and spoke with a Kiwi accent, noting that she kicked him out days later when police revealed he was a sex offender.

"He said all kinds of crazy stuff, I have no idea if any of it was true, I think I had a lucky escape getting rid of him so quickly."