MELBOURNE, Dec 5 (Reuters) - Aluminium maker Alcoa said on Monday it was assessing the impact of a power outage last week at its Portland smelter in Australia that forced it to halt a potline, adding that it was facing 'substantial challenges'.

"We are now beginning to assess the operational impact of the outage and, given the significance of this event, we have substantial challenges and analysis ahead of us. As that work is completed, we will provide an update," it said in a statement.

(Reporting by Melanie Burton; Editing by Richard Pullin)