The FBI has confirmed an imprisoned murderer who confessed to 93 murders over four decades is the most prolific serial killer in US history.

Axar.az reports citing BBC that police have matched Samuel Little, 79, to 50 cases from 1970 to 2005 thus far.

He has been serving life in prison since 2012 for the murders of three women.

Little targeted vulnerable individuals, mostly black women, many of whom lived were sex workers or drug users, officials say.

A former competitive boxer, Little would knock his victims out with punches before strangling them - meaning that there were not always "obvious signs" that the person had been brutally murdered.

Some of these deaths were never investigated by the FBI as a result, and many deaths were incorrectly determined to be overdoses or accidental. Some bodies have never been found, the agency said.

In a statement on Monday, the FBI said its analysts believe "all of his confessions are credible".

"For many years, Samuel Little believed he would not be caught because he thought no one was accounting for his victims," FBI crime analyst Christie Palazzolo said in the statement.

"Even though he is already in prison, the FBI believes it is important to seek justice for each victim - to close every case possible."

Law enforcement is still working to verify the remaining 43 cases he confessed to.