An American teenager is accused of gunning down his family at home just minutes before midnight on New Year's Eve.

The 16-year-old is in custody after his parents, sister and a family friend were shot dead in the family's New Jersey home as the New Year approached.

Police responded to a call at 11.43pm on 31 December and arrived shortly after to find the suspect's father, Steven Kologi, 44; mother Linda Kologi, 42; sister Brittany Kologi, 18; and family friend Mary Schultz, 70, deceased.

Investigators say the boy appears to have used a semi-automatic rifle to murder his victims. They did not say who the weapon belonged to.

Monmouth County Prosecutor Christopher Gramiccioni said he expected the boy to face four counts of murder.

A joint investigation by the Monmouth County Prosecutor's Office and Long Branch Police Department is ongoing.

The boy is reported to have been taken into custody without incident. As of yet, his motives have not been ascertained.

"A 16-year-old boy is in police custody after four people were found shot to death inside his parent's Long Branch home," Gramiccioni said, in a statement.

"Long Branch police responded to a 911 call reporting shots fired at 635 Wall Street in the city at 11.43 pm on 31 December.

"It appears the boy used a Century Arms semi-automatic rifle."

Anyone with information about the case is asked to call Detective Andrea Tozzi, of the Monmouth County Prosecutor's Office at 800-533-7443, or Detective Michael Verdadiero of the Long Branch Police Department at 732-222-1000.

Meanwhile in London, police have launched four separate murder investigations after knife crime claimed the lives of four different young men across the capital during the New Year's Eve celebrations.

The isolated incidents led to the deaths of two teenagers and two men in their twenties. A fifth man is in a critical condition after being attacked at one of the incidents.