Imai told reporters that the U.S. government's reviewing process and the decisions over its acquisition proposal did not seem properly handled, adding Nippon Steel is "entitled to a proper review," according to the report.
Biden cited national security concerns for rejecting the $14.9 billion bid on Friday after a year of review.
Imai said that it would not take long for the company to announce countermeasures to the U.S. government's decision, the report said.
The U.S. government move cuts off a critical lifeline of capital for the beleaguered American icon, which has said it would have to idle key mills without the nearly $3 billion in promised investment from the Japanese firm.
It also represents the final chapter in a high profile national security review, led by the Committee on Foreign Investment in the United States (CFIUS), which vets investment for national security risks and had until Dec. 23 to approve, extend the timeline or recommend Biden block the deal.
The proposed tie-up has faced high-level opposition within the United States since it was announced a year ago, with both Biden and his incoming successor Donald Trump taking aim at it as they sought to woo union voters in the swing state of Pennsylvania, where U.S. Steel is headquartered. Trump and Biden both asserted the company should remain American-owned.