Alcoa president and chief executive William F Oplinger told Wall Street on Wednesday the company was axing jobs at its Kwinana refinery, south of Perth, as it worked out what to do with the plant.
"Kwinana is a marginal asset at this point, we'll consider options on the table, including curtailment and closure," he said.
Mr Cook said the comments were "disappointing and create unnecessary uncertainty for their workforce".
"Local workers deserve better," he said.
The premier also said Alcoa had recently told government there was no plan to cease operations at the site in the near future.
"We will continue to work with Alcoa to ensure local jobs are protected and its operations can continue, while maintaining the highest environmental standards," Mr Cook said.
Alcoa said it was examining its WA operations "to improve competitiveness, mitigate impacts of lower bauxite grade from our mines, and maintain current production levels".
"Our Kwinana Alumina Refinery has been running at about 80 per cent capacity since January and we recently advised our workforce of restructuring that will remove about 90 roles by early next year," a spokesman said in a statement.
"Where possible redeployment and voluntary redundancy will be offered."
Kwinana Alumina Refinery was commissioned in 1963 and was the first of Alcoa's three WA refineries making alumina from the raw material bauxite.
It employs about 900 workers and 300 contractors.
Alcoa employs about 4500 workers in WA and Victoria and also operates numerous mines.
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