The US says new tariffs on steel and aluminum could go into effect as soon as in 15 days. On Thursday, president Trump signed off on the tariffs he announced last week. This comes as China warns about rising tensions and global fears surrounding a possible trade war. Roee Ruttenberg has more.

U.S. President Donald Trump said - no one should be surprised that he signed off on new tariffs on steel and aluminum. He said - it was a campaign promise, and he's now delivering.

DONALD TRUMP US PRESIDENT "Our factories were left to rot and to rust, all over the place. Thriving communities turned into ghost towns. Not any longer. The workers who poured their souls into building this great nation were betrayed. But that betrayal is now over."

The new duties amount to 25% on foreign steel and 10% on imported aluminum. Workers from both industries - attending the White House signing on Thursday - welcomed the move. They said they hoped it'll resurrect made-in-America metals. Their metals. But critics warn the move could spark a worldwide trade war. More than a hundred Republican lawmakers - many of them Trump supporters - sent the president a letter on Thursday urging him to reconsider.

RON JOHNSON US REPUBLICAN SENATOR "A generalized tariff that would harm allies, harm American consumers, by the way, harm American workers that use steel in production, hurting their competitive nature in global markets as well, I'm opposed to that."

Trump said the U.S.'s neighbors to the north and south -- Canada and Mexico -- will be exempt, pending ongoing trilateral trade talks. Both had heavily lobbied the administration in recent weeks. He also suggested that there might be some flexibility for Washington's other "friends". That's unlikely to include Beijing. China is the world's biggest steelmaker, and has been seen largely as the target of these tariffs, even though Chinese steel accounts for but a fraction of U.S. imports. On Thursday, Beijing warned it would make a "justified and necessary response".

WANG YI CHINESE FOREIGN MINISTER "As the world's largest two economies, China's and America's interests are deeply interwoven. We hope the two sides can sit down calmly and find a win-win solution through equal and constructive dialogue."

ROEE RUTTENBERG WASHINGTON "The lead up to this -- and the potential fallout -- have been anything but calm. In fact, Trump's top economic adviser resigned this week in protest. But the president is unfazed. He says he supports free trade. But insists it must be fair and this, he believes, is a significant step in that direction. Roee Ruttenberg, CGTN, in Washington."