Belgium's justice minister says he will stay in his job despite calls for his resignation after a prison inmate killed four people while on furlough.

Minister Koen Geens told parliament on Thursday that "to persevere when the going is tough is sometimes tougher that to simply give up."

The attacker used his two-day furlough to kill two policewomen and a bystander in Liege and an old friend outside the eastern industrial city.

The government acknowledged that the man, Benjamin Herman, had been indirectly mentioned in two radicalization reports for his links with extremist inmates.

During his rampage, he yelled "Allahu akbar," Arabic for "God is great," several times. He was known to local authorities as a repeat offender involved in petty crime and drugs.