Nippon Steel's $15 billion bid for U.S. Steel has been referred to U.S. President Joe Biden, a White House spokesman has said, giving the president 15 days to decide on a tie-up he has previously said he opposes.

The Committee on Foreign Investment in the United States (CFIUS), which reviews foreign investments in the U.S. for national security risks, referred the bid to Biden after it was unable to reach a consensus.

"We received the CFIUS evaluation and the president will review it," the White House spokesperson said. President-elect Donald Trump, set to retake the office on Jan. 20, has also opposed the deal, which was first announced last December.