By David Brunnstrom, Simon Lewis, Trevor Hunnicutt and Tim Kelly TOKYO/WASHINGTON (Reuters) -The foreign ministers of Japan and the United States said their countries' ties were stronger than ever on Tuesday,

U.S. Steel and its would-be Japanese suitor filed twin lawsuits Monday to defend their $15 billion merger. President Biden issued an order on Friday to block the deal, citing national security concerns.

U.S. President Joe Biden's decision to block Nippon Steel's $14.9 billion bid for U.S. Steel casts a shadow over Secretary of State Antony Blinken's visit to Japan on Tuesday for farewell meetings with Washington's most important ally in Asia.

U.S. President Joe Biden’s decision to reject a bid by Nippon Steel to acquire U.S. Steel on national security grounds isn’t the first time friction over trade and investment has irked Washington’s closest ally in Asia.

Japan-based Nippon Steel has offered to give the U.S. government veto power over any potential reduction in steel output by U.S. Steel if the federal government approves a proposed merger.

The Japanese firm’s planned purchase of its U.S. rival was blocked last week by President Joe Biden after a year of diplomatic tension, political debate and lobbying efforts from the companies and unions. Biden, who had previously said he opposed the tie-up, cited national-security risks, despite Japan being a close ally.

The president said he was moving to protect national security in deciding the fate of the iconic Pennsylvania-based company, which became a contentious political issue in an election year.

The companies condemned the president's decision and hinted at taking legal action, while U.S. Steel's CEO accused Biden of "political corruption."

Nippon Steel shares closed down 0.75% at 3,158 yen ($20.03) on Monday, compared with a 1% fall in broader Topix index. They settled at 3,182 yen on Dec. 30, the final trading day of 2024 on the Tokyo Stock Exchange, which was closed for the remainder of last week for the New Year holiday.

The usually reticent ally is making it clear that it will not go quietly in its fight to overturn President Biden’s decision to block Nippon Steel’s $14.1 billion takeover bid of U.S. Steel, with Prime Minister Shigeru Ishiba warning this week that the rejection could have real consequences for the bilateral relationship,

U.S. Steel and Japan’s Nippon Steel sued the United States government on Monday in a last-ditch attempt to revive their attempted merger after President Biden blocked it last week on the basis that the transaction posed a threat to national security.