WASHINGTON (Reuters) -U.S. President Donald Trump on Friday expressed support for Nippon Steel's ( NISTF ) $14.9 billion bid for U.S. Steel, saying their "planned partnership" would create jobs and help the American economy.

Shares of U.S. Steel soared 21% as investors interpreted Trump's post on Truth Social to mean Nippon Steel ( NISTF ) had received his approval for its long-planned takeover, the last major hurdle for the deal.

"This will be a planned partnership between United States Steel ( X ) and Nippon Steel ( NISTF ), which will create at least 70,000 jobs, and add $14 Billion Dollars to the U.S. Economy," Trump said in a post on Truth Social.

This week, Reuters reported that Nippon Steel ( NISTF ) has said if the merger is approved, it would invest $14 billion into U.S. Steel's operations, including up to $4 billion in a new steel mill.

Trump added that the bulk of that investment would occur in the next 14 months and said he would hold a rally at U.S. Steel in Pittsburgh next Friday.

In a statement, U.S. Steel praised Trump's leadership.

"U.S. Steel will remain American, and we will grow bigger and stronger through a partnership with Nippon Steel ( NISTF ) that brings massive investment, new technologies, and thousands of jobs," it said.

Nippon Steel ( NISTF ) said it applauded Trump's decision to approve the "partnership". The White House did not immediately reply to questions about the announcement.

U.S. Steel kept rising after hours and reached $54, just shy of Nippon Steel's ( NISTF ) $55-per-share offer price made in late 2023. While no details were released, investors expressed confidence that terms will be similar to those agreed in 2023. Investors said that eventually U.S. Steel will no longer be publicly traded and they will receive a cash payout for their shares.

The deal has been one of the most highly anticipated on Wall Street after it morphed into the political arena with fears that foreign ownership would mean job losses in Pennsylvania where U.S. Steel is headquartered. It factored into last year's election that saw Donald Trump returned to the White House.

Pennsylvania Senator Dave McCormick, who also called the deal a "partnership," on Friday said it was a "huge victory for America and the U.S. Steel Corporation," that will protect more than 11,000 Pennsylvania jobs, and support the creation of at least 14,000 more.

The last pieces of the deal came together surprisingly fast. The Committee on Foreign Investment in the U.S., which reviews deals for national security risks, told the White House this week that the security risks can be addressed, Reuters reported, moving the final decision to Trump's desk.

Following an earlier CFIUS-led review, former President Joe Biden blocked the deal in January on national security grounds.

The companies sued, arguing they did not receive a fair review process. The Biden White House rejected that view.

The companies argued Biden opposed the deal when he was running for reelection to win support from the United Steelworkers union in the battleground state of Pennsylvania. The Biden administration had defended the review as essential to protecting security, infrastructure, and supply chains.

Trump also initially opposed the deal, arguing the company must be owned and operated in the U.S.

The United Steelworkers were against the deal as recently as Thursday when they urged Trump to block the deal despite the $14 billion investment pledge from Trump.

For investors, including prominent hedge funds, the news spells relief after more than a year of waiting for a resolution. "There were huge high fives all around today," one recent investor said, adding "We understood Donald Trump's psyche and we played it to our advantage here."

Investors said Trump appears to have won ground after the pledge for new investments was increased.

"This deal ensures that steelmaking will live on in Pittsburgh for generations," another investor said.