Crews fighting a wildfire around Fort McMurray in northern Alberta say drones are buzzing about and threatening operations.
Senior wildfire manager Chad Morrison says drones haven't affected firefighting operations at this point, but can be dangerous.
"We're still investigating ... (but) my understanding is the fleet is still flying at this point," Morrison said at a briefing Thursday.
"We haven't grounded anything."
Drones, while small, can get caught up in the rotors of helicopters or plane propellers.
"Impacts and consequences can be very high, so we want to make sure that people are taking this warning very seriously."
Morrison said he believes the drones are attempting to get aerial footage of the fire.
The area around the blaze that forced more than 80,000 people from their homes is still restricted air space and "as busy as any city airport" as choppers and air tankers continue to fly in and out.
Fines for not following the rules are as high as $25,000.
The fire is holding at 5,800 square kilometres, but Morrison said most of that area is no longer burning except for some parts of the perimeter.
Scott Long of Alberta Emergency Management said the return of residents to the city has been going smoothly. About 8,000 of 13,000 evacuees eligible to go back did so Wednesday on the first day of a staged re-entry.
Organizers were expecting about 21,000 more people of an eligible 40,000 to go home Thursday.