CNN  —  President Joe Biden on Thursday said he believes it’s “vital” that US Steel remain American owned and operated, dealing a blow to the iconic company’s planned sale to Japan’s Nippon Steel. The announcement marks the strongest opposition yet from the White House to a controversial deal that has drawn some bipartisan criticism. “It is important that we maintain strong American steel companies powered by American steel workers. I told our steel workers I have their backs, and I meant it,” Biden said in a statement. “US Steel has been an iconic American steel company for more than a century, and it is vital for it to remain an American steel company that is domestically owned and operated.” The Biden administration is concerned about some of Nippon’s assets in China, a person familiar with the administration’s thinking told CNN. Neither US Steel nor Nippon were immediately available to provide comment. Previously, the Biden administration voiced support for a thorough review of the $14.1 billion deal that was announced in December. Lael Brainard, director of the National Economic Council, issued a statement that month saying Biden believes the deal deserves “serious scrutiny” in terms of its impact on national security and supply chains. Shares of Pittsburgh-based US Steel plunged 13% on Wednesday — its worst day since 2020 — after the Washington Post and other outlets reported Biden planned to express veiled opposition to the sale ahead of Japanese Prime Minister Fumio Kishida’s arrival in Washington for a state visit on April 10. US Steel shares retreated another 3% on Thursday. It’s noteworthy that Biden is voicing his opposition to the deal before the national security review that is believed to be ongoing by the Committee on Foreign Investment in the United States, or CFIUS. That inter-agency panel is led by the Treasury secretary and is supposed to weigh deals without taking politics into account. The Treasury Department declined to comment.