Security sources have told Reuters they are not ruling out the blast in the town of Ratingen was a targeted attack.

Firefighters and police were called to the building this morning after concerns were raised about two residents, the interior minister of North Rhine-Westphalia has said.

Herbert Reul added that when they forced open an apartment door, they found a fire and one of the inhabitants used an unidentified object to cause an explosion, the minister added.

State broadcaster ZDF has said a mother and son had been living in the apartment block and concerns were raised because their mailbox was overflowing.

Television footage from the scene showed police snipers positioned on a balcony across the road from the building as smoke poured out of a top-floor apartment.

Police have said a large scale operation is under way.

Ratingen is located on the northeastern outskirts of Duesseldorf, the state capital.

Interior minister Nancy Faeser is expected to give a statement on the incident this afternoon, ZDF has reported.