Two days after bomb attacks at Brussels airport and on a packed metro killed 31 people and injured hundreds, a security guard who worked at a Belgian nuclear site was killed but the local prosecutor on Saturday ruled out any militant link.

The Charleroi prosecutor's office also denied media reports that his security pass had been stolen and been de-activated as soon as investigators raised the alarm, public broadcaster VTM said.

The office declined to comment when contacted by Reuters.

Le Soir newspaper said the man was a guard at Belgium's national radioactive elements institute at Fleurus, to the south of Brussels.

A police spokeswoman said she could not comment because an investigation was ongoing.

In a nation on high alert following this week's attacks, the media reports tap into fears about the possibility militants are seeking to get hold of nuclear material or planning to attack a nuclear site.

On Thursday, DH had reported the suicide bombers who blew themselves up on Tuesday originally considered targeting a nuclear site, but a series of arrests of suspect militants forced them to speed up their plans and instead switch focus to the Belgian capital.

Late last year, investigators found a video tracking the movements of a man linked to the country's nuclear industry during a search of a flat as part of investigations into the Islamist militant attack on Paris on Nov. 13 that killed 130 people.

The video, lasting several hours, showed footage of the entrance to a home in northern Belgium and the arrival and departure of the director of Belgium's nuclear research programme.