A wildfire smothered Alberta Highway 35 on Tuesday, cutting off the province from the Northwest Territories overnight.
The Alberta government announced the closure on Tuesday afternoon at 3:30pm, and later said that closure would continue into Wednesday.
Traffic heading south is being held at Enterprise, with northbound traffic able to travel only as far as Zama City, the Alberta government said in an updated advisory on Wednesday morning.
The NWT government's Department of Infrastructure tweeted: "No vehicles are allowed to travel between Enterprise and the Alberta border due to a wildfire at Steen River, Alberta.
The Alberta government instructed all people in the Steen River area to leave immediately and head south, with a centre available in La Crete for those with nowhere to go.
There was no immediate sense of when the highway, which is a key route for both travelling NWT residents and goods heading north, might reopen.
The road remained closed overnight, with highways personnel manning barricades.
The Alberta government said the wildfire had crossed Highway 35 north of Steen River late on Tuesday afternoon.
A message on the Alberta highways website earlier in the day read: "Highway 35 closed approximately 7 km north of Steen River, north of High level - fire near the highway affecting traffic.
"Visibility in the area may be suddenly reduced to less than 1 km in smoke."
High Level, the nearest Alberta community of significant size, remains off-limits due to its own nearby wildfire, which triggered an evacuation order for the community last week.
The wildfire near Steen River, initially reported to be 166 hectares in size, was said to have grown to 17,000 hectares and was out of control on Wednesday morning, according to Alberta Agriculture and Forestry. Around 30 people were said to be fighting the fire on Tuesday.
Meanwhile, the NWT government said another wildfire had emerged on the territory's side of the border, some 59 km south of Enterprise.
"This is a lightning-caused fire ... 3.5 hectares in size and is being held," the territory's Department of Environment and Natural Resources said. "Fire crews continue to action the fire."
At Indian Cabins on Tuesday afternoon – within the closure zone, on the Alberta side of the border – the temperature was recorded at higher than 33C.
The road closure immediately led some NWT residents to express concern about the availability of goods in northern communities, with Highway 35 – which becomes Highway 1 in the NWT – a vital road artery.
The NWT can also be accessed by road through BC, otherwise deliveries would need to arrive either by air or by rail to Hay River – though the rail line runs parallel to the highway, and would presumably face similar disruption.