UPDATE 1-Norsk Hydro CEO says securing alumina to replace Alunorte output is priority

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OSLO/LONDON, March 15 (Reuters) - Norsk Hydro Asa sees securing alternative sources of alumina to replace reduced supplies at its Brazilian plant as a priority, CEO Svein Richard Brandtzaeg told Reuters.

Brazilian authorities in late February ordered Norsk Hydro's Alunorte plant, the world's largest alumina plant, to cut output by half until it addressed pollution concerns, leading the Norwegian company to declare force majeure with no time set to restart production.

Alunorte refines bauxite to alumina, which is turned into aluminum at huge smelters. It produces 5.8 million metric tonnes of alumina per year, according to Norsk Hydro's website.

In a telephone interview, Brandtzaeg said he could not yet say when full production could resume but dialog with the Brazilian authorities was "very constructive."

He said Alunorte had supplies to last into April but sourcing extra volumes was a priority.

"We are looking at alternative sourcing on the international market," he said. "Sourcing extra volumes is a key priority."

Hydro's shares closed down 1.6 percent on Thursday. (Reporting By Terje Solsvik in Oslo and Barbara Lewis in London Editing by Susan Fenton)