Also taking aim at Seoul, Trump complained that America's 2012 free trade deal with South Korea "was a disaster". Most Democrats voiced support for the president's action on metals, and Republicans, with the exception of Senator Rob Portman of OH, urged caution. "It is in accordance with worldwide law", said Li Guanghui, vice-president of the Chinese Academy of global Trade and Economic Cooperation in Beijing.
But in Trump's first year as president the trade deficit swelled to a record high of $375.2 billion by the US's counting.
"Part of the options would be tariffs coming in".
"I look at it two ways: I want to keep prices down, but I also want to make sure that we have a steel industry and an aluminum industry, and we do need that for national defense", Trump said. Alcoa Inc shares ended 1.4 percent higher, with Century Aluminum up 0.5 percent.
The Commerce Department formally submitted the results of its investigations into steel and aluminum imports to the White House in January.
The Commerce Department has not revealed its recommendations following the inquiry.
Republican Senator Pat Toomey, who represents Pennsylvania - a state with storied connection to the USA steel industry - told Trump the tariffs could "invite retaliation" and would be hard to justify on national security grounds.
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"I've been looking at them for two years as I went around campaigning", Trump said.
At the same time, some Democrats from states Trump carried - and the more populist voters who propelled him into office - are pushing for actions that resemble the president's trade threats.
"I think we do need to be careful here that we don't start a reciprocal battle on tariffs", Senator Roy Blunt, a Missouri Republican told Trump.
Several lawmakers cited the need to maintain USA energy independence and the electrical grid. By putting tariffs on imports, it effectively raises the price of imports so that domestic steel and aluminum producers aren't being undercut. "You may have a higher price, but you have jobs", Mr. Trump said.
Meanwhile the ministry said on Monday that it had uncovered instances of styrene dumping by USA and South Korean manufacturers in an initial ruling of an ongoing trade investigation into the chemical.
"Trump is today more aware that the impulsive actions he has threatened to take would carry some very significant economic and political costs".
The U.S. remains the world's top major power, with "unparalleled" military strength, foreign ministry spokesman Geng Shuang told reporters at a regularly scheduled news briefing.
A Chinese government think-tank warned earlier that "unreasonable" US trade actions on steel will be met with "countermeasures" under World Trade Organization rules.