Three people were killed and one badly injured in a shoot-out on Saturday near the Jewish Museum in Brussels city centre, Belga news agency said, quoting firefighters' emergency services.
"I am shocked by the murders committed at the Jewish museum, I am thinking of the victims I saw there and their families," Belgian foreign minister Didier Reynders said on Twitter.
A bystander, Alain Sobotic, told AFP he had seen the minister at the scene.
No information was immediately available from police on the circumstances of the shoot-out but Sobotic said he had seen two corpses just inside the doors of the museum, a young woman and a man.
The Jewish Museum of Belgium, which was not answering calls, is located in the heart of the Sablon district which is home to the city's top antique dealers.