UPDATE 2-Norsk Hydro CEO says securing alumina to replace Alunorte output is priority

priority@ (Adds company putting 400 Brazil employees on 15-day leave)

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, Chief Executive Svein Richard Brandtzaeg told Reuters.

Brazilian authorities ordered Norsk Hydro's Alunorte plant, the world's largest alumina plant, to cut output by half in late February 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 aluminium 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 would resume but dialogue 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."

The company said in a statement on Thursday it had put 400 employees that work in its facilities in the northern state of Pará on a compulsory 15-day leave beginning on April 2.

Hydro's shares closed down 1.6 percent on Thursday. (Reporting By Terje Solsvik in Oslo, Barbara Lewis in London and Marta Nogueira in Rio de Janeiro Editing by Susan Fenton and Tom Brown)