Five people are missing following an explosion at a German industrial park for chemical companies.

The blast in Leverkusen has been declared an “extreme threat” by authorities and left several people injured.

The explosion happened at 9.40am local time (7.40am GMT) and caused a fire at a fuel depot at Chempark, an industrial park for chemical companies including Bayer and Lanxess.

Chempark operator Currenta said several staff were hurt, with at least two seriously injured and five missing.

It is not yet clear what caused the explosion, Currenta added.

Sirens and emergency alerts on the German civil protection agency’s mobile phone app warned citizens of “extreme danger”.

Police in nearby Cologne said they did not have any information about the cause or size of the explosion, but a large number of police, firefighters, helicopters and ambulances had been deployed to the scene.

Residents have been asked to remain inside their homes, keeping doors and windows closed.

Currenta said air conditioning systems should also be turned off while it measured the air around the site for possible toxic gas.

Motorways nearby have been closed with drivers told to take detours and avoid the area.

Daily Koelner Stadt-Anzeiger reported the explosion took place in the Buerrig neighbourhood at a rubbish incineration plant of the chemical park with the smoke cloud moving in a north-western direction toward the towns of Burscheid and Leichlingen.