US President Donald Trump said Friday that he would double steel import tariffs to 50 percent, speaking in Pennsylvania at a US Steel plant where he also touted a partnership between the American steelmaker and Japan's Nippon Steel.

"We're going to bring it from 25 percent to 50 percent, the tariffs on steel into the United States of America, which will even further secure the steel industry," he said.

"Nobody's going to get around that," he added in the speech before blue-collar workers in the battleground state that helped deliver his election victory last year.

The doubling of levies will take place next week, said the White House in a social media post.

Since returning to the presidency in January, Trump has imposed sweeping tariffs on allies and adversaries alike in moves that have rocked the world trade order and roiled financial markets.

He has also targeted sector-specific goods including steel, aluminum and automobiles with 25 percent tariffs.

On Friday, Trump mounted a defense of his trade policy, arguing that tariffs helped protect the US company. 

He added that the plant would not exist if he did not also impose duties on metals imports during his first administration.

In his speech, Trump stressed as well that despite a recently announced partnership between US Steel and Nippon Steel, "US Steel will continue to be controlled by the USA."

He added that there would be no layoffs or outsourcing of jobs due to the deal.

A proposed $14.9 billion sale of US Steel to Nippon Steel had previously drawn bipartisan opposition, and former president Joe Biden blocked the deal on national security grounds shortly before leaving office.

The terms of the new partnership remain murky, however.

The United Steelworkers union (USW) which represents thousands of hourly workers at US Steel facilities, said in a statement Wednesday that the "partnership" announcement "continues to raise more questions than answers."

"Nippon still maintains it would only invest in USS facilities if it owned the company outright. We've seen nothing in the reporting to indicate that position has changed," the USW statement added.

Trump said previously that US Steel would remain in America with its headquarters to stay in Pittsburgh, adding that the arrangement with Nippon would create at least 70,000 jobs and add $14 billion to the US economy.

But union leaders said they had no confirmation of how much of the $14 billion would go towards union-represented sites, if any.

Trump had opposed Nippon Steel's takeover plan while on the election campaign trail, but since returning to the presidency, he signaled that he would be open to some form of investment after all.