Japan, he said, would stick to World Trade Organization rules in terms of taking measures.
"Europe is certainly not a threat to American internal security so we expect to be excluded", Malmstrom told reporters before speaking at a conference in Brussels.
Trump signed an order for 25 percent tariffs on steel imports and 10 percent for aluminum at the White House on Thursday, but exempted Canada and Mexico. Instead, they said, Trump is bolstering domestic industries in violation of global trade rules. "Big Deficit. If not, we Tax Cars etc. FAIR!"
"The European Union, wonderful countries who treat the United States very badly on trade, are complaining about the tariffs on Steel & Aluminum", Trump tweeted.
Japan, the US's top economic and military ally in Asia, was next in line.
EU Commissioner Cecilia Malmstrom, who coordinates policy for the world's biggest trading bloc, said she shared USA concerns about overcapacity in the steel sector but did not believe in tariffs as a way to solve the problem.
The EU exported about 5.5 million tons of steel to the USA previous year.
The American president made his comments after crunch talks in Brussels between European Union negotiators and US Trade Representative Robert Lighthizer in an effort to defuse a bitter row that many fear could turn into an all-out trade war.
Brussels has reminded Trump that tit-for-tat trade measures deepened the Great Depression in the 1930s and in the 2000s cost thousands of USA jobs when Washington imposed tariffs on European steel.
Shortly after Trump's tweet, Australian Prime Minister Malcolm Turnbull said on Twitter that the relationship between the two allies was fair and reciprocal.
The EU and Japan past year formally agreed the broad outlines of a landmark trade deal that was announced as a direct challenge to the protectionism championed by Trump. China accounts for only a small fraction of U.S. steel imports, but its rapid rise to produce half the world's steel has helped create a global glut that has driven down prices.
Beijing vowed to "firmly defend its legitimate rights and interests".
While carrying a message to Washington to push forward a diplomatic breakthrough over North Korea, South Korea's national security office chief Chung Eui-yong asked USA officials to support Seoul's request for a waiver, a presidential spokesman said.
China's metals industry issued the country's most explicit threat yet in the row, urging the government to retaliate by targeting US coal - a sector that is central to Trump's political base and his election pledge to restore American industries and blue-collar jobs.