US President Joe Biden speaks during a campaign event in Milwaukee, Wisconsin
US President Joe Biden speaks during a campaign event in Milwaukee, Wisconsin AFP

President Joe Biden said on Thursday he opposed the proposed sale of US Steel to Japan's Nippon Steel, in a move that could annoy Washington's close Asia-Pacific ally Tokyo.

Biden's intervention comes less than a month before he is due to host Japan's Prime Minister Fumio Kishida for a state visit to the White House to underscore ties.

"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 deal remains under US federal security review.

Biden's statement comes as the Democrat tries to woo US unions ahead of November's presidential election, in which he will compete for working class voters with Republican former president Donald Trump.

US Steel is based in Pittsburgh in the key battleground state of Pennsylvania, which Biden will need to win again as he did in the 2020 election if he is to beat Trump.

The two companies unveiled Nippon's planned $14.1 billion acquisition of Pittsburgh-based US Steel in December but it immediately faced opposition from several quarters.

The White House said at the time that the deal should be closely investigated, warning that it could have national security implications.

The firms in December asked the Committee on Foreign Investment in the United States (CFIUS) -- an interagency body established to review foreign takeovers of US firms -- to evaluate the deal.

Unions have reacted with fury to the proposed deal, despite the combined company's vow to honor contract agreements between US Steel and the United Steelworkers (USW) union.

In December the USW ripped the proposed deal as reflective of a "greedy, shortsighted attitude" of US Steel, which dates to 1901, and questioned the ability of Nippon to honor contracts.

At the time the transaction also drew bipartisan howls on Capitol Hill, with Pennsylvania Democratic Senator John Fetterman calling the deal "absolutely outrageous."

Trump-allied Ohio Senator JD Vance and two other Republicans asked Treasury Secretary Janet Yellen, who chairs CFIUS, to block the deal, calling domestic steel production "vital to US national security."