WASHINGTON: Shares of steelmaker US Steel skyrocketed 21% on Friday (May 23) after President Donald Trump announced his support for a "partnership" with Nippon Steel. 

"US Steel will REMAIN in America, and keep its Headquarters in the Great City of Pittsburgh," Trump said in a Truth Social post, adding that the "planned partnership" between America's US Steel and Japan's Nippon Steel would create at least 70,000 jobs and add US$14 billion to the US economy. 

It was not immediately clear what the terms of this new partnership were, and neither company responded to a request for comment.

Trump's remarks are the latest in a long saga which began in December 2023, when US Steel and Nippon Steel announced plans for a US$14.9 billion merger.  

The deal was originally meant to close by the end of 2024's third financial quarter, but was held up by former president Joe Biden, who blocked it in his last weeks in office on national security grounds.

The two firms then filed a lawsuit against the Biden administration's "illegal interference" in the transaction.

Earlier this month, Trump ordered his own review of the deal, directing the government's Committee on Foreign Investment in the United States (CFIUS) to look into the proposed acquisition.

CFIUS, tasked with analyzing the national security implications of foreign takeovers of US companies, was given 45 days to submit its recommendations to Trump.

The United Steelworkers' union, which represents US Steel employees, voiced its concern about the deal on Thursday, citing reports that CFIUS had issued its recommendations to the president.

"Allowing the sale of US Steel to Nippon, a serial trade cheater, will be a disaster for American Steelworkers, our national security and the future of American manufacturing," USW International President David McCall said in a statement.

"It is simply absurd to think that we could ever entrust the future of one of our most vital industries — essential to both national defense and critical infrastructure — to a company whose unfair trade practices continue to this day," he added.