U.S. President Donald Trump has announced plans to double the current 25-percent tariffs on steel and aluminum imports to 50 percent starting Wednesday, saying the move will "further secure" the American steel industry.

Speaking at a rally in Pennsylvania on Friday, Trump said the original 25-percent tariff had an institutional "loophole," and stressed that the new rate would be impossible to evade.

The surprise announcement comes amid speculation that Trump will approve a merger between U.S. Steel and Japan's Nippon Steel. He described it only as a "planned partnership" and called it a "blockbuster agreement."

Trump reassured the American public that U.S. Steel will remain an American company and keep its headquarters in Pittsburgh.

He said that, like his predecessor Joe Biden, he initially opposed the sale over national security concerns. But he later recognized how strongly Japan wanted to invest in the U.S., noting that Nippon plans to invest 14 billion dollars.

The increased tariffs are expected to deal a blow to the South Korean steel industry, with exports to the U.S. accounting for about 13 percent of nation's total steel shipments last year.