TOKYO (Kyodo) -- Japan's Nippon Steel Corp. and United States Steel Corp. said Monday they have filed two lawsuits seeking to protect their merger plan from "illegal and improper political and anticompetitive interference" after it was blocked by U.S. President Joe Biden last week.

The announcement came as Japanese Prime Minister Shigeru Ishiba warned of potential repercussions from Biden's decision for the close business ties between the two countries and urged a detailed explanation of the president's reasoning for barring the $14.1 billion deal, which he has said creates national security risk.

Regarding the lawsuit filed in the U.S. Court of Appeals for the District of Columbia Circuit, the two steelmakers said in a statement that Biden's order was made for "purely political reasons which are irrelevant to, and are to the detriment of, U.S. national security."

They asserted Biden "ignored the rule of law to gain favor with" the United Steelworkers union, referring to the key supporter of his Democratic Party that has opposed the sale of the struggling Pittsburgh-headquartered producer to the major Japanese company.

They also accused a panel of U.S. federal agencies tasked with looking into the deal before Biden made his decision of failing to conduct "a good faith" regulatory review.

The second lawsuit was filed against rival Cleveland-Cliffs Inc., its CEO Lourenco Goncalves and United Steelworkers union leader David McCall at the U.S. District Court for the Western District of Pennsylvania. It alleges they colluded to block the acquisition of U.S. Steel by any party other than Cleveland-Cliffs, the second-largest American steelmaker.

Nippon Steel and U.S. Steel described the actions as an attempt to monopolize the U.S. domestic steel market, stating the lawsuit was filed to prevent "further collusive and anticompetitive behavior" that has caused them billions of dollars in damages.

Nippon Steel said its Chairman Eiji Hashimoto will hold a press conference Tuesday morning.

While the fight to defend the deal intensifies, Ishiba said he takes "seriously" the concerns among Japanese business circles over future investments between Japan and the United States.

"I will strongly urge the U.S. government to respond toward dispelling (the concerns)," he said during his first press conference of the year on Monday.

He also said, "It is out of the question if we don't get proper explanation on why there are concerns on national security."

Biden's order, issued Friday, marked the first time a U.S. president has blocked a merger and acquisition deal involving a Japanese company. He said the takeover would "create risk for our national security and our critical supply chains."

The order required Nippon Steel and U.S. Steel to abandon their merger plan within 30 days.

Biden, who leaves office on Jan. 20, and President-elect Donald Trump have both publicly opposed the deal announced in December 2023 by Japan's biggest steelmaker and the third-largest U.S. producer.

"Why would they want to sell U.S. Steel now when tariffs will make it a much more profitable and valuable company? Wouldn't it be nice to have U.S. Steel, once the greatest company in the world, lead the charge toward greatness again?" Trump said on his Truth Social platform on Monday. "It can all happen very quickly!"

In a statement issued hours before the announcement of the lawsuits, Nippon Steel President Tadashi Imai said, "We will pursue every possible measure" as "we will never give up on expanding our U.S. operations."

The outgoing president's decision has drawn criticism from the Japanese business community, with some insisting that he has set a bad precedent for other domestic firms pursuing acquisitions in the United States, a key ally of Japan.

The Washington Post reported Sunday that Biden's decision to block Nippon Steel's bid overrode the advice of numerous top aides, including Secretary of State Antony Blinken, U.S. Ambassador to Japan Rahm Emanuel and Treasury Secretary Janet Yellen, who argued against or expressed reservations about the president's position.

Japan's Economy, Trade and Industry Minister Yoji Muto also told reporters Monday that the president's decision is "extremely regrettable and hard to understand."

"Economic relations serve as the foundation of bilateral ties. I believe it is very crucial to develop an environment where companies can make investment decisions without concerns," Muto said.