A gunman killed two police officers and a passer-by on Tuesday before being shot dead in the centre of the Belgian city of Liege, public broadcaster  Radio Television Belge Francophone  (RTBF)  said on its website and authorities said terrorism could not be ruled out as a motive.

La Libre Belgique newspaper quoted a police source as saying the gunman shouted Allahu Akbar -- God is greatest in Arabic -- and RTBF said investigators believed the attacker may well have had a terrorist motive.

The man had taken a woman hostage at some point in the attack, the newspaper said.

The national anti-terrorist crisis centre, which Interior Minister Jan Jambon said on Twitter was monitoring the situation, said terrorism could not be excluded as a motive though it was also looking into other possible reasons.

"It (terrorism) is one of the questions on the table, but for the moment all scenarios are open," a spokesman for the crisis centre said.

Two other police officers had been injured, Belga news agency said. A spokeswoman for the Liege public prosecutors office confirmed that two police officers had been shot dead and the gunman "neutralised" but could not confirm other details.

Images on social media showed people scurrying for safety on Liege's central boulevard d'Avroy with shots and sirens being heard in the background.

Liege, an industrial city close to the German border in a French-speaking region, was the scene of a shooting in 2011, when a gunman killed four people and wounded more than 100 others before turning the gun on himself.

Belgium has been on high alert since a Brussels-based Islamic State cell was involved in attacks on Paris in 2015 that killed 130 people and Brussels in 2016 in which 32 died.