LEVERKUSEN, Germany: At least 16 people were injured and five missing after an explosion at a chemical park in Leverkusen, western Germany, officials said Tuesday, urging residents to stay indoors as a huge column of black smoke billowed from the site.

The blast happened at around 09:40 am (0740 GMT) "for still unknown reasons", Chempark operator Currenta said in a statement.

Germany's NINA warning app sent an "extreme danger" alert to residents, telling them to close all doors and windows and keep the emergency numbers free as much as possible.

Currenta said the incident occurred at Chempark's landfill and waste incineration area in Leverkusen's Buerrig district, which is separate from the main industrial park that houses numerous chemical companies including Bayer, Lanxess and Evonik Industries.

The city of Leverkusen said on Facebook that 16 people were injured in the incident, four of them seriously.

Currenta had earlier said "several employees" were hurt.

Both Currenta and Leverkusen authorities spoke of five people still missing.

According to city officials, the blast led to a fire at a solvent storage tank that took several hours to put out, with firefighters from nearby Cologne called in to help with the efforts.

Large numbers of police, firefighters and rescue crews were deployed to the scene, as well as pollution detection experts amid concerns that the smoke coming from the site could pose a health risk.

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

They urged drivers to steer clear of the area and repeated the message that residents should move indoors "and close all windows and doors as a precaution".