Police and soldiers backed by armoured vehicles and helicopters have been scouring a nature reserve for three days to find Jurgen Conings.

The 46-year-old soldier is thought to be well-armed and was already on a watch-list for suspected right-wing extremists when he absconded after stealing weapons from his military base.

"There was a failure in procedures," Defence Minister Ludivine Dedonder told public broadcaster RTBF.

She vowed to seek a change in the law to allow a more thorough vetting of any soldier granted access to sensitive sites and weapons -- like the rocket launchers found in Conings' abandoned car.

Separately, Prime Minister Alexander de Croo has ordered a report into extremism within the ranks Belgium's police, armed forces and customs officers and will present an "action plan" next month.

Conings is assumed to present an "acute threat" of violence and is thought to have take several more weapons from the base where he served as an instructor.

He is reported to have left behind letters suggesting that he may pose a threat to state representatives and public figures -- including renowned virologist Marc Van Ranst.

Van Ranst has become a media figure as an expert commentator during the coronavirus crisis and a target of attacks from conspiracy theorists and the Flemish far-right.

Dedonder said Conings had been disciplined and had a security clearance revoked because of hateful views he had expressed on social media, including racism and threats of violence.

But, as an instructor charged with preparing recruits for overseas missions he still had access to a dangerous arsenal of military weapons, she admitted.