"The potential for escalation is real as we have seen from the initial responses", he said.
On Monday morning, Trump tweeted that Canada and Mexico would only be exempt from the tariffs, which consist of a 25 percent tax on steel imports and a 10 percent tax on aluminum imports, if they take Trump-friendly steps on NAFTA and drug trafficking.
They are "safeguard" tariffs, a kind of emergency shield erected to stop a sudden, unforeseen and damaging import surge that could seriously damage a particular industry. German producers account for about 4% of USA steel imports.
French President Emmanuel Macro has blasted the move as "economic nationalism" - adding it was a war in which all sides lose.
"We have large trade deficits with Mexico and Canada". Both Canada and Mexico send more than 75 per cent of their goods exports to the United States.
Trump's surprise vow to impose a new steel tariff rocked global markets.
Syrians flee government advances in eastern Ghouta; Douma convoy approved
Opposition groups launched a counteroffensive Sunday, sending fighters behind regime lines in a series of attacks. The UN Security Council demanded a 30-day countrywide ceasefire a week ago, but this has failed to take effect.
South Korea shipped 3.6 million tons of steel products to the US previous year, becoming the No. 3 steel exporter after Canada and Brazil, according to the US Department of Commerce.
This point was repeated on Monday by White House Director of Trade and Industrial Policy Peter Navarro, who told Fox News, "As the president said, we can't have a country without an aluminum and steel industry. Tariffs on Steel and Aluminium will only come off if new & fair Nafta agreement is signed", Trump tweeted on Monday.
Trump has long criticized the North American Free Trade Agreement.
Refusing to spare friendly countries from the punishing tariffs could further inflame tensions created by Trump's sudden announcement on Thursday to protect American metal producers on national security grounds.
It said the plan will spark retaliation from the affected countries and inflate the cost for US construction materials for enterprises and automakers, which will eventually hurt American consumers.
"I understand the frustration, but the medicine the United States administration is willing to use is not right", the former Finnish prime minister said.
"There is always retaliation, and typically a lot of these countries single out agriculture when they do that". The controversial move is likely to invite retaliatory measures from foreign countries, raising the specter of a trade war between the United States and several other steel-producing countries.
The protectionist policy will be made official in the next two weeks, White House officials said Sunday, as the administration defended the decision from critics in Washington and overseas.
On Friday, Trump defended the proposed tariffs, arguing that in certain circumstances, "trade wars are good".