Fire crews scrambled late into the night on Saturday as fierce winds whipped flames beyond Canadian Forces Base Wainwright and into the surrounding rural area.
The death toll was not immediately clear but a doctor at a hospital in Edmonton, 180 km northwest, told Canada's national broadcaster that patients were being brought to hospitals across the city.
He said health-care facilities around the base were "completely overwhelmed" by the number of people needing care.
"This number of patients would overwhelm the city of Edmonton," Warren Thirsk, an emergency department physician at the Royal Alexandra Hospital, told CBC News.
"It is more patients than any of us want to count right now. It's a disaster. It's a one in every 15 to 25-year disaster."
Defense Minister David McGuinty said the Royal Canadian Mounted Police were investigating the incident and described the situation as "very, very active and delicate."
He said authorities were still working to determine what happened and expected to provide further information.
The Royal Canadian Mounted Police said officers were called shortly after 4 pm Saturday following reports of a fire in a training area at the base.
By 11 pm, the Municipal District of Wainwright said on social media the fire was being held and that property and critical infrastructure were protected. There was no evacuation order for residents.
McGuinty, speaking to reporters in Ottawa, confirmed there were casualties but had no other details to share.
"I cannot recall such an incident in my short time for sure as a minister, but I cannot recall an incident like this in recent memory," McGuinty said.
"We will have more to say in the coming hours and days. This is an active and delicate situation and soon to be an active and delicate investigation."
In a social media post, Canadian Prime Minister Mark Carney said "Our thoughts are with all the brave women and men affected".
Carney said the federal government was working closely with Alberta officials and local authorities.
Canadian Conservative opposition leader Pierre Poilievre said he had spoken with Carney and the defence minister to offer support to those affected by the fire.
"Pray for Wainwright, everyone," Poilievre said in a social media post. He urged people in the area to follow emergency instructions and keep roads clear for first responders.
The fire came days before the province was set to vote on beginning a process that could lead to its seeking independence from Canada.
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