By Steve Holland and Svea Herbst-Bayliss

WASHINGTON (Reuters) -U.S. President Donald Trump on Friday expressed support for Nippon Steel's $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 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 and Nippon Steel, 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 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.

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 that brings massive investment, new technologies, and thousands of jobs," the company said in a statement.

Nippon Steel also applauded Trump's decision. "The partnership is a game changer - for U.S. Steel and all of its stakeholders, including the American steel industry, and the broader American manufacturing base," the Japanese company said in a statement on Saturday.

A Nippon Steel spokesperson in Tokyo declined to comment on the $14 billion investment and the 14-month timeline that Trump cited.

The White House did not immediately respond to questions about the announcement. It is unclear whether Trump's term "partnership" refers to the full acquisition Nippon Steel has been pursuing.

For Japan's top steelmaker, the deal is core to its global expansion strategy, lifting production to 86 million metric tons from 63 million tons now - especially at a time when domestic demand is declining.

The merger would create the world's third-largest steel producer by volume, following China's Baowu Steel Group and Luxembourg-based ArcelorMittal, according to World Steel Association data.

U.S. Steel kept rising after hours, hitting $54, just shy of the $55 per share that Nippon Steel offered 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.