TOKYO, Jan 15 (Reuters) - Japan's Nippon Steel remains interested in working with the incoming administration of Donald Trump to try to seal a takeover of U.S. Steel , its vice chairman Takahiro Mori said an opinion piece in the Wall Street Journal.

Last week, Nippon Steel and U.S. Steel filed two lawsuits after U.S. President Joe Biden blocked a $14.9 billion buyout of the American steelmaker by the Japanese firm. President-elect Donald Trump takes office on Monday.

Enforcement of Biden's order, which gave the parties 30 days to unwind the transaction, was postponed until June after the companies sued the U.S. president, alleging he violated the constitution by depriving them of due process when he blocked the deal.

"Nippon Steel and U.S. Steel will do whatever it takes to close this transaction," Mori said in the WSJ piece. "We believe our case is strong, and we look forward to our day in court."

Cleveland-Cliffs , whose earlier bid for U.S. Steel was rejected by the latter's board, is partnering with peer Nucor to prepare a potential all-cash bid for the company again, a source told Reuters this week.

"We remain interested in exploring possible partnerships with the new administration to invest in and grow U.S. Steel to benefit American workers, customers, and national security," Mori, Nippon Steel's key negotiator on the deal, said in the opinion piece.

The decision to file lawsuits was not taken lightly, Mori said, while reiterating that Japan is one of U.S. closest allies and the company did not believe there was any national security concern regarding the takeover.

"Major companies in allied nations want to invest in the U.S. and employ Americans. Now they wonder if they'll be treated as partners or political pawns," Mori said.

(Reporting by Katya Golubkova; Editing by Sonali Paul)

((jekaterina.golubkova@thomsonreuters.com;))