US President Donald Trump said on 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.

Since returning to the White House in January, Trump has imposed sweeping tariffs on allies and adversaries alike.

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

On Friday, Trump said that tariffs helped protect the US company, adding that the plant would not exist if he did not also impose tariffs on metals imports during his first administration.

In his speech, Trump also stressed that "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. (AFP)