German officials Tuesday asked residents in the west-central city of Leverkusen to stay in their homes and close their windows and doors after an explosion at a chemical plant.

Germany’s Federal Office for Civil Protection and Disaster Assistance issued a warning of “extreme danger” following the explosion early Tuesday at the Chempark industrial site in Leverkusen, just outside Cologne. Officials were concerned about air quality in the vicinity following the blast, which they say occurred in an area of the park where waste is incinerated.

Pictures posted to social media showed huge clouds of smoke billowing from the site and flames.  Police closed several nearby roads as a precaution.

In a statement, Currenta, operator of Chempark, said the explosion caused a fire in a tank farm adjacent to the incineration area. They said several workers were injured and five were missing. Rescue crews were on the scene. The company set up a hotline for residents to call for information about loved ones who may have been in the blast.

The company said the cause of the explosion is unknown. Officials said they would hold a news conference later in the day.

Chempark’s website says its Leverkusen location hosts more than 30 large companies including Covestro, Bayer, Lanxess and Arlanxeo.

Some information for this report came from Reuters and the Associated Press. 
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