The announcement came hours after Iran said it was suspending some limits set by a multinational nuclear deal -- on the one-year anniversary of Trump's pullout from the accord.

The White House said it was imposing sanctions on all trade of Iran's iron, steel, aluminum and copper, the country's biggest export after oil -- which the United States is already working to halt completely.

"Today's action targets Iran's revenue from the export of industrial metals -- 10 percent of its export economy -- and puts other nations on notice that allowing Iranian steel and other metals into your ports will no longer be tolerated," Trump said in a statement.

"Tehran can expect further actions unless it fundamentally alters its conduct."

But in a switch in tone, Trump, who talked tough on North Korea before entering landmark talks with leader Kim Jong Un, said he was also willing to negotiate.

"I look forward to someday meeting with the leaders of Iran in order to work out an agreement and, very importantly, taking steps to give Iran the future it deserves," he said.