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.

At least 16 people were injured and five remain missing.

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

The city of Leverkusen said in a statement that the explosion at the Chempark site, about 13 miles north of Cologne on the Rhine river, occurred in storage tanks for solvents.

It said four people were severely injured and 12 less seriously. Five people are missing.

Currenta, the company operating the chemical park, said the explosion happened at 9.40am local time at the storage tanks of their waste management centre and then developed into a fire.

“Sirens were operated to warn residents and warning alerts were sent,” Currenta said in the statement.

A dark cloud of smoke rises above the Chempark in Leverkusen, Germany (Oliver Berg/dpa via AP)

Police in nearby Cologne said a large number of officers, firefighters, helicopters and ambulances from across the region had been deployed to the scene.

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 roads.

Daily Koelner Stadt-Anzeiger reported that the smoke cloud was moving in a north-western direction toward the towns of Burscheid and Leichlingen.