U.S. President Donald Trump on Thursday launched a trade probe against China and other exporters of cheap steel into the U.S. market, raising the possibility of new tariffs and sending shares of some U.S. steel makers up over 8%.

Citing concerns about national security, Mr. Trump made the announcement at a White House ceremony with U.S. steel executives from Nucor Corp., United States Steel Corp. and TimkenSteel Corp. alongside Commerce Secretary Wilbur Ross, a billionaire businessman who made part of his fortune investing in the steel business.

“Steel is critical to both our economy and our military,” said Mr. Trump. “This is not an area where we can afford to become dependent on foreign countries.” Mr. Trump won many votes in industrial States like Michigan and Pennsylvania with a pledge to boost manufacturing and crack down on Chinese trade practices.

National security

The unusual step of launching an investigation comes as Mr. Trump is pressuring China to do more to rein in an increasingly belligerent North Korea.

Diverging from the Obama administration's approach to the issue, which relied largely on filing complaints to the World Trade Organization (WTO), Mr. Trump ordered a probe under Section 232 of the Trade Expansion Act of 1962, which lets the President impose restrictions on imports for reasons of national security.