UPDATE 1-Warnig to replace Cheung as RUSAL board chairman

* Warnig on Rosneft board, Nord Stream executive

* RUSAL says Cheung has no disagreements with the board

* Chairmanship in conflict since Vekselberg resignation

(Adds background on RUSAL chairmanship)

MOSCOW, Oct 1 (Reuters) - Matthias Warnig, a former East German secret agent turned investment banker who has known Russian President Vladimir Putin since the 1990s, has been appointed chairman of Russia's RUSAL , replacing Barry Cheung, who resigned.

Cheung, the chairman of the Hong Kong Mercantile Exchange, will remain on the board as an independent director, RUSAL, the world's largest aluminium company, said in a statement.

"Mr. Cheung confirmed that he has no disagreements with the board, nor are there any matters relating to his resignation as the chairman of the Board that need to be brought to the attention of the shareholders of the company," RUSAL said.

Warnig headed Dresdner Bank's operations in Moscow more than a decade ago. He now holds a board seat at state-controlled Rosneft and is a top executive at Nord Stream, a strategic gas pipeline from Siberia's gas fields to Europe.

The chairmanship of RUSAL, which produces about 10 percent of the world's primary aluminium and is coping with weak prices and relatively high debts, has been surrounded by conflict since the resignation of Viktor Vekselberg, its largest shareholder after tycoon Oleg Deripaska.

Cheung was appointed in March to replace Vekselberg, who accused management, headed by Deripaska, of plunging the company into a "deep crisis". RUSAL responded by threatening to sue Vekselberg.

At the crux of the dispute is RUSAL's valuable 25 percent stake in top world palladium and nickel miner Norilsk Nickel , which Deripaska is reluctant to sell.

(Writing by Melissa Akin; Editing by Gleb Bryanski and Catherine Evans)

((melissa.akin@thomsonreuters.com)(+7 495 775 1242)(Reuters Messaging: melissa.akin.thomsonreuters.com@reuters.com))