A bushfire that triggered an emergency warning and forced the closure of Canberra's airport has been downgraded.

The ACT Emergency Services Agency is urging residents to be vigilant as the blaze may still be difficult to control under current conditions.

Earlier the Kallaroo Road and Redwood Forest fires in the northern suburb of Piallago formed what has been dubbed the Beard fire, spanning 147ha as of 2pm on Thursday.

People in Beard, Harman, Oaks Estate and Crestwood were told to seek shelter immediately with authorities warning it is was too late to leave.

Arrivals and departures are affected due to aviation firefighting operations.



Please check with your airline for up to date flight details.



Please follow ACT ESA to keep updated. https://t.co/V0uMtBAFJs — Canberra Airport (@CanberraAirport) January 23, 2020

Aircraft are water-bombing and monitoring the fireground.

People in Pialligo and Brindabella Business Park, near the airport, have been advised to stay up to date and monitor conditions.

The fire is travelling in a southeasterly direction towards Beard, Harman, Oaks Estate and Crestwood (Queanbeyan), the ACT Emergency Services Agency said.

"The fire is out of control. Firefighters are on scene. However, under current conditions, the fire is difficult to control."

AAP

Defence chief Angus Campbell said some workers at the Brindabella Business Park may need to be evacuated.

"The defence force is both assisting to a degree and looking to whether that needs to be reinforced," he told reporters.

"I have people who are both involved as persons who need to be moved from areas and office buildings that are potentially in danger, and also those persons who are part of the (Operation) Bushfire Assist effort."

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The blaze started on Wednesday but strong winds and high temperatures have seen conditions in Canberra deteriorate.

ACT emergency services have issued a total fire ban.

A 93,650ha fire near Adaminaby, south of the ACT, is at "emergency" level, as is the Clyde Mountain fire on the NSW south coast.