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PRAGUE (Reuters) - An explosion killed six people at a refinery in the Czech Republic on Thursday, rescue officials said.

Two other people were taken to hospital with serious injuries, a fire department spokeswoman told Czech Television.

The blast occurred at Unipetrol’s plant in Kralupy nad Vltavou about 30 km (19 miles) north of Prague.

“There was an explosion at the storage tank but no subsequent fire in the Kralupy refinery,” Unipetrol spokesman Pavel Kaidl said. “The situation is under control and there is no other danger. There was no leakage of dangerous substances.”

The mayor of Kralupy told Czech Television the blast occurred during cleaning of the storage tank.

Unipetrol had been preparing the refinery for maintenance and an upgrade scheduled to run from March 27 to May 9. The facility has an annual capacity of 3.2 million tonnes of oil.

In August 2015, a blast and fire at Unipetrol’s Litvinov facility knocked the refinery out for more than a year.

In February, Poland’s PKN Orlen said it had raised its stake in Unipetrol to more than 90 percent from 63 percent, meeting a threshold to take full control and delist the company from the Prague stock exchange.