The firm has agreed to be taken over by Japan's Nippon Steel in a $14.9 billion deal.

But the U.S. president now says that's not a move he supports.

On Thursday, Biden said the country needed to "maintain strong American steel companies powered by American steelworkers".

He called the firm "iconic", and said it should always be domestically owned and operated.

The White House says Biden spoke to the boss of the United Steelworkers union to say he had their back.

He has courted labor organisations as a key constituency in this election year.

It all comes after some U.S. lawmakers said the deal deserved "serious scrutiny".

Some have said it threatens jobs and the supply of steel in the country.

The opposition could spell trouble for Nippon Steel, which has pledged to pay the U.S. firm a $565 million breakup fee if it fails to complete the takeover.

Biden's comments could also overshadow his upcoming summit with Japanese Prime Minister Fumio Kishida.

On Friday, Nippon Steel said the deal would not immediately mean layoffs or plant closures.

It said it remained determined to complete the takeover.