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Update, 3.10pm: A suspicious fire burning in Middle Swan is "out of control and unpredictable", authorities say.

An advice warning has been issued for people in the southern part of Middle Swan in the eastern part of Stratton. People in the area are being urged to stay up to date, close all doors and windows and turn off evaporative air conditioners.

A total of 40 firefighters from 12 stations and three helitacs are battling the blaze, which was reported just before 11am and is burning in grassland north of Roe Highway.

The fire is contained but not under control and there is no threat to lives or homes in the area. The Department of Fire and Emergency Services says the fire is moving north at a moderate speed.

The fire has burnt through about 20 hectares and started in grass and scrub north of Roe Highway, between Great Northern Highway and the railway line.

Roe Highway is closed between Great Northern Highway and Toodyay Road and Great Northern Highway is closed between Roe Highway and Oakover Road.

Motorists are being advised to avoid the area.