At least 11 people were being treated at hospitals in Superior, where the blast happened, and nearby Duluth, Minnesota.

The Associated Press

SUPERIOR, Wis. — A tank containing crude oil or asphalt exploded at a large refinery in Wisconsin on Thursday, injuring several people and prompting fire officials to urge people living near the still-burning plant to evacuate their homes.

No fatalities have been reported, but at least 11 people were being treated at hospitals in Superior, where the blast happened, and nearby Duluth, Minnesota.

The explosion at the Husky Energy oil refinery happened at about 10 a.m., Superior Fire Chief Steve Panger told The Associated Press.

Panger said the fire was out by 11:20 a.m., although smoke could still be seen rising from the plant. Superior police later tweeted that the fire had reignited and urged residents living within a one-mile radius of the refinery to leave. Police blocked roads around the refinery in Superior, which is home to about 27,000 people and borders Minnesota to the north and the tip of Lake Superior.

Essentia Health spokeswoman Maureen Talarico said five people injured in the explosion are being treated at St. Mary’s Medical Center, a Level II trauma center in Duluth. She said emergency room physicians described those patients as awake and alert. Another five are being treated at St. Mary’s Hospital in Superior, Talarico said. She said the extent of injuries is unknown.

In Duluth, St. Luke’s Hospital was treating one person and did not expect to receive any more, spokeswoman Jessica Stauber said. She did not have a condition for that person.

Panger said a small tank containing either crude oil or asphalt exploded in the refinery. A contractor who was inside the building told WDIO television that the explosion sounded like “a sonic boom” and that it happened when crews were working on shutting the plant down for repairs.

No damage estimate was available.

Calgary-Alberta-based Husky Energy refinery bought the refinery from Indianapolis-based Calumet Specialty Products Partners last year for over $490 million. It’s Wisconsin’s only refinery, and it produces gasoline, asphalt and other products.

The refinery, which dates back to the early 1950s, has a processing capacity of around 50,000 barrels per day and a storage capacity of 3.6 million barrels of crude and products. It processes both heavy crude from the Canadian tar sands in Alberta and lighter North Dakota Bakken crude.