Motorists have been warned to avoid a section of Mitchell Freeway after a fatal crash on Wednesday night.

The crash happened about 8.16pm on Mitchell Freeway northbound in Butler.

At least one person has died but it is not yet known if there has been other fatalities.

Police, St John WA ambulances, and firefighters remain at the scene. Major crash investigators are also at the crash site.

Both north and southbound lanes have been shut between Romeo Road and Butler Boulevard.

Main Roads WA warned motorists there was congestion and to exercise extreme caution.

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