The biggest wildfire Alberta firefighters have ever faced continues to rage out of control in the province's northeast.

The Richardson backcountry fire, located about 60 kilometres north of Fort McMurray, started on May 15 — the same day as the Slave Lake wildfire that destroyed hundreds of homes and businesses.

The Richardson fire is now similar in size to Prince Edward Island, having scorched 570,000 hectares of muskeg and bush.

Almost 700 firefighters battling the massive blaze were joined by 40 more, who arrived from Mexico Sunday night.

Ironically, the fire is burning during one of the slower fire seasons in the province, said Alberta fire information officer Rob Harris.

This spring has seen fewer fires than the five-year seasonal average, he said.

But any fire burning on May 15 became an inferno as four days of high winds, some reaching speeds of 100 kilometres per hour, fanned flames northward, Harris said.

Unlike the Slave Lake fire, which forced the evacuation of 7,000 people from their homes, the Richardson fire continues to pose little significant threat to human settlement or property.