Police urged local residents to close their windows and doors and avoid using emergency telephone numbers if possible

Leverkusen (Germany) (AFP) - One person has died and four are missing after an explosion at a chemical park in western Germany, the site operator said on Tuesday, urging residents to stay indoors as a column of black smoke billowed from the area.

Another 31 people were injured in the incident at the Chempark complex in the city of Leverkusen, according to police, five of them seriously.

The cause of the huge blast, which was heard several kilometres away, remains unknown.

Germany’s NINA warning app sent an “extreme danger” alert to residents, telling them to close their doors and windows and keep emergency numbers free as much as possible.

The explosion happened at around 09:40 am (0740 GMT) at Chempark’s waste incineration site in Leverkusen’s Buerrig district, according to site operator Currenta.

The area is separate from the main industrial park that houses numerous chemical companies including Bayer, Lanxess and Evonik Industries.

At an afternoon press conference in Leverkusen, Chempark head Lars Friedrich said the warning to stay indoors remained in place while experts raced to identify the composition of the smoke cloud.

Asked whether the cloud might contain toxic gases, Friedrich declined to speculate but said nothing could be ruled out.

City mayor Uwe Richrath said it was “a dark day for the people of Leverkusen”.

Playgrounds in the Buerrig and Opladen neighbourhoods would be closed, he said. Residents were also advised not to eat fruit and vegetables from their garden for now.

- ‘Tragic accident’ -

The city of Leverkusen said on its Facebook page that measurements of pollution levels were all still “in the green” and that any measures taken were as a precaution.

“We are deeply saddened by this tragic accident and the death of an employee,” said Chempark’s Friedrich, adding that “rescue efforts continue at full speed” to find the four missing employees.

The blast had triggered a fire in tanks used as storage for liquid solvents awaiting incineration, Friedrich said.

The large blaze took several hours to put out, with firefighters from nearby Cologne called in to help.

Three of the tanks “were completely or partially destroyed”, Friedrich said, making it impossible to tell for now where the explosion started.

Large numbers of police, firefighters and rescue crews were deployed to the scene, as well as pollution detection experts.

Police in Cologne tweeted that they had closed several motorways in the area because of “major damage” and urged drivers to avoid the area.

Locals shared images on social media of the black cloud rising into the air, with some saying their windows were rattled by the force of the explosion.

According to a report in Der Spiegel magazine, the blast was measured as far as 40 kilometres (25 miles) away.

- Massive chemicals complex -

Leverkusen, on the eastern bank of the Rhine river, lies about 20 kilometres north of Cologne in the western state of North Rhine-Westphalia. It has a population of more than 160,000 people.

According to Currenta, the Chempark chemicals complex is one of the largest in Europe – with more than 70 companies based at its three sites.

The chemical incident comes as Germany is reeling from historic floods earlier this month that left at least 180 people dead in North Rhine-Westphalia and neighbouring Rhineland-Palatinate state.

The high death toll raised questions over the country’s warning systems, and whether enough had been done to notify residents ahead of time.

Government officials have called for old-school sirens to be used more widely again, on top of smartphone warning apps and radio and TV broadcasts.

Currenta said it had used both sirens and the NINA app to sound the alarm after the explosion.