-China trade deal was in the offing.
He also claimed that since imposing tariffs at the start of March 2018, U.S. markets were "up as much as 21%".
Meanwhile, Trump has cast doubt on the agreement by threatening Mexico with tariffs to force the country to stem the flow of migrants to the U.S.
Speaking in London where he's attending a North Atlantic Treaty Organisation summit, Trump said he had "no deadline" for making a deal, suggesting there may not be an agreement this year, as previously hoped.
There were also hopes that the U.S. and China might strike a limited "phase one" deal to avert the 15 December round of tariffs, which will affect a wide range of consumer goods, including Apple products, clothing and Christmas lights. "Markets go up in stairs and then on the way down, it's an elevator".
It will be "definitely risk-off across the screen", Tongli Han, chief investment officer at Deepblue Global Investment, said in an interview with Bloomberg TV.
The Brazilian government said it's in touch with the US Trade Representative's office and other agencies about Trump's surprise decision, according to a source familiar with Brasilia's reaction.
The US imported almost 169,000 tons of steel from Argentina past year, representing more than $220 million in trade. "We'll see what happens", he said.
On Saturday, China said its manufacturing sector expanded in November for the first time in seven months, providing a boost to investors looking for signs of optimism in the world's second biggest economy.
The dollar weakened against a basket of key trading partners' currencies.
Trump, who had said in September that he did not need a deal before the 2020 election, sought on Tuesday to put pressure on Beijing.
Just like that, clients get exposure to a developed US$2 trillion market with a stable government and one more thing: a gold and silver haven.
"I'd fade the correction today", Eli Lee, head of investment strategy at Bank of Singapore, told Bloomberg TV.
But OANDA senior market analyst Jeffrey Halley said: "It is hard to see the USA swallowing a very bitter trade pill like that; it would, in effect, remove all of the US's leverage in the far more difficult comprehensive trade negotiations to come".
Brazil's economy, foreign affairs and agriculture ministries said in a joint statement they will "defend Brazil's trade interests and assure the flow of trade with the U.S".
There may be some massive initial market swings in store, said Chris Weston, head of research at Pepperstone Group Ltd., in a note to clients.
Currently, the futures on the S&P 500 are reporting a 0.11% gain.
She said farmers are aware of the monumental changes the Trump administration is trying to achieve with China and are focused instead on trying to secure other deals, like the US-Mexico-Canada Agreement, which the three countries completed a year ago but has yet to pass Congress. "In fact, it probably makes the relationship more hard to manage, because we've taken tariffs off the table".
"The president's objective always has been to get the right deal independently of when, or anything else like that".