By Andrea Shalal

WASHINGTON, April 22 (Reuters) - The United Steelworkers union told Treasury Secretary Scott Bessent it remains "unalterably" opposed to a $15 billion bid by Japan's Nippon Steel 5401.T to buy U.S. Steel X.N, or any deal that would give Nippon influence over the company.

USW President David McCall told Bessent in a letter sent late on Monday that the union did not believe any mitigations offered during an initial Committee on Foreign Investment in the United States (CFIUS) review of the deal answered its deep national security and economic concerns.

"We remain unalterably opposed to the acquisition of United States Steel $(USS.NZ)$ by Nippon Steel Corporation," McCall wrote in the letter, a copy of which was viewed by Reuters.

The Treasury department did not have an immediate comment.

McCall said the union's concerns had deepened since former President Joe Biden blocked the deal in January, noting that Nippon continued to engage in practices that fueled overcapacity in global markets, including through 1 million tons of capacity it held in China through joint ventures.

He added that the U.S. Commerce Department this month assigned a preliminary 205% tariff rate to Nippon in an earlier administrative review of Non-Oriented Electric Steel (NOES) from Japan.

McCall said Nippon also appeared poised to divest certain U.S. Steel assets and had failed in various meetings to commit to maintaining production at plants in Pennsylvania's Mon Valley of Pennsylvania and elsewhere.

U.S. President Donald Trump, who took office for the second time on January 20, began his term noting that he "wouldn't mind" if Nippon Steel took a minority stake in U.S. Steel, implying he would seek an overhaul of the deal structure.

Last month he directed CFIUS, which reviews foreign investments for national security risks, to take a fresh look at the all-cash bid for U.S. Steel to help determine if "further action" would be appropriate.

Trump has repeatedly said he does not think a foreign company should control U.S. Steel, and the two firms say they are working closely with his administration to "secure a significant investment."

(Reporting by Andrea Shalal; Editing by David Gregorio)

((andrea.shalal@tr.com; +1 202-815-7432;))