Firefighters are battling an out-of-control blaze threatening lives and properties in WA's South West.

A bushfire watch and act alert was issued for those east of Scott River in the Shire of Nannup on Thursday afternoon, and warned residents they must either prepare to evacuate or prepare to defend their properties.

The fire started in the local nature reserve just after midday, and the warning is active for people within the vicinity of two kilometres south of Pagett Nature Reserve, bounded by Dennis Rd, Governor Broome Road and Avery Road, in the locality of Scott River East.

The fire is considered out-of-control, unpredictable and is travelling in a south-westerly direction.

Governor Broome Road between Dennis and Avery roads have been closed as a result of the blaze,and motorists have been asked to avoid the area.