Fire and smoke from the massive inferno in southeastern Oregon is creating a phenomenon known as Pyrocumulus clouds (‘fire clouds’) - dangerous plumes of smoke and ash that can reach up to six miles into the sky. The Pyrocumulus clouds are “easily visible from 100 to 120 air miles away”, authorities said on Friday.

       The Bootleg fire grew to around 377 sq Miles (976 sq kilometres), an area larger than New York City.

       Commander Rob Allen, a frontline firefighter told AFP news agency, said: “The Bootleg Fire perimeter is more than 200 miles long - that's an enormous amount of line to build and hold.

       “We are continuing to use every resource, from dozers to air tankers to engage where it's safe to do so especially with the hot, dry, windy conditions predicted to worsen into the weekend.”

       The massive blaze has been burning for more than a week and remains only seven percent contained, according to fire officials.

       Nearly 2,000 firefighters are currently responding to the inferno which has destroyed 21 homes and prompted a series of evacuations across the state.

       Forestry officials believe the wildfire is the state’s fifth largest blaze in more than a century.

       The fire comes as the state experiences a devastating heatwave which threatens to create new ignition points in the tinder-box landscape.

       Fire authorities say the Pyrocumulus clouds are forming between 3-5pm each day as the sun pierces the smoke, heats the ground and creates an updraft of hot air.

       READ MORE: US fires map: Apocalyptic wildfires burning area size of Connecticut

       Officials said: “If these clouds ‘collapse’ they can cause dangerous outflow winds and ember falls for firefighters working in the area.”

       Meteorologists have warned pyrocumulus clouds could precede the formation of the more deadly pyrocumulonimbus cloud - dubbed by NASA as the “fire breathing dragon of clouds” - which is capable of creating its own weather.

       In a statement on Thursday, Joe Hessel, incident commander for the Oregon Department of Fire, said the fire is likely going to grow even further.

       He said: “This fire is going to continue to grow - the extremely dry vegetation and weather are not in our favour.

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       “We are going to continue to prepare lines, protect structures, and move resources as we can around the fire’s edge.”