Arsonists set fire to Belgium's National Institute of Criminology in Brussels early this morning, causing an explosion but no casualties, a Brussels prosecutor said.

Ine Van Wymersch said there were no immediate indications that the fire at the institute, which was empty at the time, was a militant attack although nothing had been ruled out.

Europe has been on high alert after Islamic State attacks in Paris and Brussels over the past year.

"It was arson, deliberate arson at the laboratory of the federal police," she said. "With a fire you get explosions, but it's not that explosives were thrown inside or installed."

"It is a path we are looking down," she said, referring to the possibility of militant involvement. "But certainly not the first one we are thinking about. We are thinking more of deliberate arson by organised crime. We have no indications that it was terrorism," she said.

The institute is linked to the Belgian ministry of justice and carries out forensic investigations in criminal cases, according to its website.

The attack was carried out by more than one person, van Wymersch said. Two people had been detained, although they were not necessarily the perpetrators.

Belgian broadcaster RTL said that a car rammed through barriers at the centre at about 3 am local time.