• U.S. President-elect Donald Trump has pledged to block Japanese company Nippon Steel's planned purchase of U.S. Steel.
• "I am totally against the once great and powerful U.S. Steel being bought by a foreign company, in this case Nippon Steel of Japan," Trump said in a post to his social media platform Truth Social on Monday night Eastern time.
• "As President, I will block this deal from happening," he said, adding that he will make U.S. Steel "Strong and Great Again" through the use of tax incentives and tariffs. 

U.S. President-elect Donald Trump has pledged to block Japanese company Nippon Steel's planned purchase of U.S. Steel.

"I am totally against the once great and powerful U.S. Steel being bought by a foreign company, in this case Nippon Steel of Japan," Trump said in a post on his social media platform Truth Social on Monday night Eastern time.

"As President, I will block this deal from happening," he said, adding that he will make U.S. Steel "Strong and Great Again" through the use of tax incentives and tariffs. 

While he made similar statements on the 2024 campaign trail, it was the first time that Trump had spoken about the deal since he won a second presidential term last month.

Nippon Steel, the fourth-largest global steelmaker, reached an agreement to acquire U.S. Steel last December. However, the deal encountered opposition from the United Steelworkers, a prominent labor union, as well as U.S. President Joe Biden, who has vowed that U.S. Steel will remain American-owned.

The Committee on Foreign Investment in the U.S., which examines possible national security risks of transactions by foreign entities, has been reviewing the deal.

U.S. Steel has an annual production capacity of around 20 million metric tons, while Nippon Steel is the leading steel producer in Japan. Together, the two companies would have a total capacity of up to 86 million tons.

"Nippon Steel is determined to protect and grow U. S. Steel in a manner that reinforces American industry, domestic supply chain resiliency, and U.S. national security," Nippon Steel said in an email statement.

"We will invest no less than $2.7 billion into its unionized facilities, introduce our world-class technological innovation, and secure union jobs so that American steelworkers at U. S. Steel can manufacture the most advanced steel products for American customers," a representative shared.

U.S. Steel did not immediately respond to CNBC's requests for comment on Trump's latest remarks.

A spokesperson for Nippon Steel previously said the acquisition would revitalize the American Rust Belt and enhance U.S. national security "in a way no alternative can."

Takahiro Mori, Nippon Steel's vice chairman, had told reporters in November that he believes the deal could be closed before Trump assumes the White House in January. In September, U.S. Steel CEO David Burritt had also expressed confidence, defending the planned sale as a deal that would strengthen national security, as well as economic and job security. 

Shares of U.S. Steel have fallen over 15% year to date, while Nippon Steel's shares slipped more than 4% across the same period of time, according to data from LSEG.