A blast followed by a fire rocked an industrial park in the western German city of Leverkusen Tuesday, sending a cloud of dark smoke towards the sky and leaving several injured. 

Germany's Federal Office for Civil Protection and Disaster Assistance classified the explosion at the Chempark site operated by Currenta as "an extreme threat,'' according to Germany's dpa news agency.

Currenta said the explosion occurred at a tank farm that is part of its facility and that five employees were missing, two were seriously injured and several others were injured as well.

Chempark urged residents of Leverkusen, which lies 20 kilometers (13 miles) north of Cologne, to "go indoors and keep doors and windows closed."

The Kölner Stadt-Anzeiger newspaper reported the incident occurred at a garbage incineration plant in the chemical park, located in the Bürrig neighborhood.

Police in Cologne tweeted that they were closing several motorways because of "major damage."

Firefighters and pollution detection vehicles have been dispatched to the scene. 

Residents said the blast could be heard from some distance, with some reporting that the force of the explosion had rattled their windows.

It is unclear what caused the blast.

Bayer, one of Germany's largest chemical manufacturers, is headquartered in Leverkusen.