AMSTERDAM, Feb 20 (Reuters) - Elliott Investment Management, U.S. billionaire Paul Singer’s activist hedge fund firm, has built up a 3.04% stake in Dutch insurer NN Group, a filing by the Dutch financial regulator AFM showed on Thursday. Elliott’s move sent NN Group shares up 2% at 0815 GMT in Amsterdam, making them the biggest winner in the blue chip AEX index. In a statement, Elliott said the investment reflected its belief “in the material and sustainable value-creation opportunity that exists at the company”.