Elliott Management has built a roughly $1 billion stake in miner Anglo American in recent months, a person familiar with the matter told CNBC.

Anglo American shares were up 5% on the news Friday.

News of the activist’s position comes as the $43 billion, London-based miner faces takeover interest from BHP, a mining giant with worldwide operations. Anglo rejected BHP’s $39 billion takeover offer Friday morning, saying BHP’s offer “significantly undervalues Anglo American and its future prospects.”

Anglo American declined to comment. The company is one of the largest producers of copper in the world.

BHP’s bid would have called for the spinoff of Anglo American’s South African subsidiaries. The metal plays a critical role in the worldwide transition to clean energy, and is used in the manufacture of EVs and wind turbines.

Anglo American chairman Stuart Chambers noted in the company’s rejection that copper represented “30% of Anglo American’s total production, and with the benefit of well-sequenced and value-accretive growth options in copper and other structurally attractive products,” there was more value to be created for shareholders in the months and years ahead.

Elliott’s position was first reported by Bloomberg.

Elliott has extensive experience with mining companies, including at suitor BHP in 2017. The activist urged the company to spin off its petroleum businesses and ditch its primary listing in Australia in favor of one in London.

BHP initially rejected those proposals, before exiting the oil business in 2021. The Australian miner is one of the largest in the world, with a market cap of $145 billion and mining sales of $53.8 billion in 2023.

CNBC’s Karen Gilchrist contributed reporting.