A warming trend this week is putting firefighters on edge in northern Alberta.

"What we're seeing is temperatures rising to 23 C and the air moisture dropping to 15 per cent, and this in wild land firefighting is called a crossover," said Barry Shellian, wildfire information officer with Alberta Agriculture and Forestry.

"When this happens, we see an increase in fire behaviour. The next few days with this weather we're going to see the wildfire intensity increase as the forest dries."

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The Fort McMurray wildfire, still out of control and growing, showed "extreme activity" in the southern parts of the wildfire on the weekend.

"Yesterday was a very challenging day for us, probably one of our most challenging days," Shellian said Monday. "We saw significant growth both on the south and west sides."

Firefighters had to leave the line for safety reasons and aircraft were grounded because of the smoke, Shellian said.

The bulk of the fire, which has burned about 285,000 hectares, is moving towards the northeast and is now 10 to 12 km from the Saskatchewan border.

"The direction of the fire spread right now is mainly into the wildlands, fortunately away from most of the communities, but there may be some oil and gas facilities in the area," Shellian said.

Lightning is expected in the area this week as well.

About 1,100 wildland firefighters, 46 helicopters and 29 air tankers — available as needed — are fighting the Fort McMurray fire.

An out-of-control wildfire burning about 10 km north of Fox Creek has forced the mandatory evacuation of 100 to 200 people living in or near Little Smoky in the Municipal District of Greenview.

That fire grew to 800 hectares from 60 on Sunday.