TOKYO -- Nippon Steel's plan to buy U.S. Steel has entered a new phase with the companies' declaration of a full-scale legal war against the U.S. government. They hope the lawsuits will overturn President Joe Biden's order to give up the acquisition.

Nippon Steel Chief Executive Eiji Hashimoto says the top Japanese steelmaker is standing firm on its proposed acquisition and poses no security risks.

The companies argued in a lawsuit announced on Monday that Biden violated the Constitution by blocking the merger via a sham national security review that deprived the companies of their right to a fair process.