Japan's top steelmaker, Nippon Steel, is sticking to its plan to close a deal by year-end to buy U.S. Steel, which it expects to boost output and profits, the company said on Thursday, despite resistance to the transaction in the U.S.

In December, Nippon Steel offered nearly $15 billion to take over iconic U.S. Steel, drawing resistance from both President Joe Biden and Donald Trump, his likely challenger in the Nov. 5 election, as well as the United Steelworkers (USW) union.

"U.S. Steel products will remain mined, melted and made in America and will continue supplying further sophisticated steel products to American industry," Nippon Steel said.