Boston gangster James 'Whitey' Bulger has been killed behind bars shortly after he was transferred to a federal prison in West Virginia. He was 89.

Bulger was listed as transferred today to USP Hazelton, a high-security prison with an adjacent minimum security satellite camp in Bruceton Mills.

He was killed inside the prison, two sources told the Boston Globe.

A fellow inmate with Mafia connections is being investigated in the homicide, the sources said.

The FBI has said that Bulger served as an FBI informant as far back as 1975, though he always denied the claim.

The organised crime boss had been convicted in 2013 of killing at least 11 people and was serving a life sentence.

He was one of the FBI's most wanted fugitives for 16 years until his 2011 arrest in Santa Monica, California.

Boston-based reporter Michele McPhee was the first to break news that Bulger had been killed.

Richard Heldreth, the president of the corrections officers' union at Hazelton, told WVNews that a male inmate had been slain there overnight, but was unable to immediately confirm the inmate's identity.

Bulger had recently been moved Hazelton from a prison in Florida after a stint in a transfer facility in Oklahoma City.

Bureau of Prisons officials and his lawyer declined last week to comment on why he was being moved.

In the past three months, there have been three homicides at Hazelton, with the officers' union blaming chronic under-staffing.

'We are very understaffed,' he told WPRI12.

Bulger was serving a life sentence after being convicted in 2013 of a litany of crimes, including participating in 11 murders.

These 1953 file Boston police booking photos show James Bulger after an arrest.

An FBI photo found in Boston shows James Bulger.