An explosion at an industrial park for chemical companies shook the German city of Leverkusen on Tuesday, sending a large black cloud rising into the air.

Germany's Federal Office for Civil Protection and Disaster Assistance classified the explosion as “an extreme threat” and asked residents to stay inside and keep windows and doors closed, German news agency dpa reported.

Operators of the Chempark site in Leverkusen, about 20 km (13 miles) north of Cologne on the Rhine river, said the cause of the explosion was unclear.

They said on Twitter that firefighters and pollution detection vans had been deployed.

Police in nearby Cologne said they did not have any information on the cause or size of the explosion and were not aware of any injuries at this point. They asked all residents to stay inside and warned people from outside of Leverkusen to avoid the region.

They also shut down several nearby major highways.

Daily Koelner Stadt-Anzeiger reported that the explosion took place in the Buerrig neigbborhood at a garbage incineration plant of the chemical park.

The paper reported that the smoke cloud was moving in a northwestern direction toward the towns of Burscheid and Leichlingen. It said firefighters from all over the region had been called in to help extinguish the fire.

Leverkusen is home to Bayer, one of Germany's biggest chemical companies.