“I am totally against the once great and powerful US 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!!!”

Nippon Steel hopes to close the deal before Trump retakes the White House on Jan 20, despite opposition from president Joe Biden and a powerful US labour union.

Last week, Japanese prime minister Shigeru Ishiba wrote to Biden, who has referred the deal to a government panel that reviews foreign investments for national security risks, asking him to approve the transaction, sources have said.

The deadline for that committee on foreign investment in the US (CFIUS) review is next month. CFIUS could approve the deal, possibly with measures to address national security concerns, or recommend that the president block it. It could also extend the review.