A national security review of Nippon Steel's $15 billion bid for United States Steel is ongoing and U.S. President Joe Biden will see what it yields before making a decision on whether to block it, the White House said on Tuesday, cautioning he still opposes the tie-up.

The statement comes after shares of U.S. Steel tumbled more than 10% on Tuesday afternoon following a report suggesting the deal would be killed in short order.

The Committee on Foreign Investment in the United States (CFIUS), a powerful panel charged with reviewing foreign investments in U.S. firms for national security risks, has until Dec. 22 to make a decision on whether to approve, block or extend the timeline for the deal's review.