U.S. President-elect Donald Trump said on Monday a series of tax incentives and tariffs will make U.S. Steel strong again, as he reiterated his opposition to Nippon Steel's planned purchase of the company.

A United Steelworkers sign is seen outside the Great Lakes Works United States Steel plant in River Rouge, Michigan U.S., September 16, 2024. REUTERS/Rebecca Cook

"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 wrote on his social-media platform Truth Social.

"I will block this deal from happening. Buyer Beware!!!"