The trustee of two of the world's largest pension funds is ratcheting up pressure on the Pebble copper and gold prospect, urging a large mining company to cut its ties to the project over what he calls environmental risks and threats to shareholders.

In a letter Monday, California Treasurer John Chiang called on First Quantum executives to "immediately undertake all measures necessary to sever any connections – financial or otherwise – with the Pebble Project or its owner Northern Dynasty Minerals."

Chiang noted he was California treasurer in an earlier divestment campaign against Pebble, when he and then-New York City Comptroller John Liu urged mining giant Rio Tinto to back out in 2013. Rio Tinto abandoned the project in 2014, leaving Northern Dynasty, a small Canadian firm, without a development partner at the time.

Chiang is trustee for the California Public Employees' Retirement System and California State Teachers' Retirement System, with assets totaling $570 billion, including long-term investments in First Quantum, one of the world's largest copper mining companies.

In December, First Quantum announced a framework agreement with Pebble calling for an initial $38 million investment to support the project's efforts to win environmental permitting, with the potential for much greater future investment.

To address longstanding environmental concerns, late last year, Northern Dynasty filed a scaled-down project under review by the U.S. Army Corps of Engineers.

Mike Heatwole, a Pebble spokesman, said the California treasurer's office has "never spoken with us" and has "no idea what our project is all about. Their characterization of risk is wrong and we will be formally extending them an invitation for a full briefing," he said.

Chiang's letter came one business day after EPA Administrator Scott Pruitt expressed concern about the project's risks to the environment and said he would keep the agency's proposed development restrictions against the mine in place, though they could later be changed or revoked.

Shares in Northern Dynasty, traded on the New York Stock Exchange, fell 21 percent on Monday.

John Tumazos, a mining securities analyst at John Tumazos Very Independent Research in New Jersey, said he doesn't think either of the recent announcements will have much long-term impact on the project.

The price of copper and project economics are more important, he said. Also, Pruitt said Friday the project can still advance through the permitting process with the Corps.

Under that process, "Pebble is dealing with rational engineers at the Corps who will consider all permutations," Tumazos said.

The Alaska Miners Association also reacted to Pruitt's statements this week, saying the EPA was "pandering to anti-development organizations."