A teenager accused of fatally stabbing a service station worker in an 'ISIS inspired' attack has been segregated for allegedly threatening to rape and kill a prison guard 'in the name of Allah'.

The 16-year-old boy and another juvenile, aged 15, were last month charged with murdering Caltex worker Zeeshan Akbar, 29, at Queanbeyan, in southern New South Wales.

They were arrested in nearby Canberra over the death of the Pakistani national, after they had allegedly written 'IS' in blood on the front of the service station.

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Two boys aged 15 and 16 were charged with fatally stabbing a Caltex service station worker

Caltex worker Zeeshan Akbar, 29, a Pakistani national was fatally stabbed doing his job

The older boy is being held at the Frank Baxter Juvenile Justice Centre at Kariong, on the NSW Central Coast north of Sydney, where he allegedly repeatedly threatened to rape and murder prison officers, the Sunday Telegraph reports.

Prison sources told the newspaper the accused murderer had also threatened to stab and rape the officers' families.

The report said he was being investigated for previously posting extremist material online as part of his possible links with Islamist groups.

His alleged behaviour has led to calls from the Public Service Association, which represents prison guards, for purpose-built juvenile detention wing of the Supermax prison at Goulburn.

The other young boy is being held in a separate juvenile justice centre and is reportedly co-operating with staff.

Both teenagers reportedly should 'Allahu Akbar' following his arrest, which is an Arabic phrase for 'God is the greatest'.

One of the teenagers accused of fatally stabbing a Caltex worker in an ISIS-inspired attack

One of the teenagers who is accused of killing Pakistani national Zeeshan Akbar in April