HARRISBURG, Pa. (AP) -- The White House said Monday that it intervened under the terms of a three-month-old national security agreement with Japan-based Nippon Steel to block a decision to stop processing raw steel at U.S. Steel's Granite City Works plant in Illinois.

In a statement, the White House said Commerce Secretary Howard Lutnick delivered the message that President Donald Trump would exercise his so-called "golden share" power that was a key element in the Trump administration's decision to allow Nippon Steel to buy out U.S. Steel.

That provision helped resolve the Trump administration's national security concerns in Nippon Steel's proposed buyout of the iconic American steelmaker and gives the federal government a say in certain company decisions involving domestic steel production, including over closing or idling U.S. Steel's plants.

"The administration did intervene to block U.S. Steel's decision to idle the Granite City Works plant, with the threat of the president invoking his golden share authority," the White House said.

Earlier this month, Pittsburgh-based U.S. Steel said it would stop processing steel slabs at its Granite City Works plant in southern Illinois, just outside St. Louis, and "optimize" its operations by focusing on processing at its Mon Valley Works in Pennsylvania and Gary Works in Indiana.

That decision was viewed as just a matter of time after U.S. Steel stopped producing steel at Granite City Works in 2023 when it idled the last operating blast furnace there. It had idled the other blast furnace there in 2019. It did say that it wouldn't lay off any of the roughly 800 workers there or reduce their pay.

On Friday, U.S. Steel reversed course and said it would continue to supply raw steel slabs to Granite City "indefinitely" and that it had "found a solution to continue slab consumption at Granite City."

It did not say what that solution was. It also declined to discuss the White House's assertion that it had blocked the move.

"The Trump administration is a great friend to the American steel industry, and we have ongoing positive and productive conversations with them on a variety of topics, but we do not plan to discuss the details of any of those conversations," U.S. Steel said in a statement.

The United Steelworkers union -- which had opposed the buyout by Nippon Steel -- accused U.S. Steel of trying to "wiggle out" of commitments that Nippon Steel made in its deal with the White House.

U.S. Steel responded that it is in full compliance with Nippon Steel's agreement with the White House.

Under the national security agreement, protections expire in 2027 for Granite City Works, but last until 2035 for U.S. Steel's other facilities.

Granite City Works makes rolls of sheet steel for the construction, container, pipe and automotive industries.

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