Eyre Highway has been reopened 20 kilometres south of Cocklebiddy in Western Australia's south-east near the South Australian border after a bushfire forced its closure.

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An emergency warning was issued earlier today to travellers crossing the Nullarbor, advising the fire had closed the highway between the Madura and Caiguna Roadhouses.

The warning has since been downgraded to an alert and the road has been reopened.

However motorists have been advised to be extremely careful and to avoid the area if possible.

Firefighters have built containment lines to control the blaze and there are no residential areas near where it is burning.

The Department of Parks and Wildlife said the fire, which started in the Nuytsland Nature Reserve, was in the vicinity of the Tommy Graham Cave.

All roads and tracks within the reserve remain closed.

The blaze was started by lightning last Friday.

Authorities said a second bushfire burning in the area had also been contained and controlled.