US President Donald Trump plans to offer Canada and Mexico a 30-day exemption from planned tariffs on steel and aluminum imports, which could be extended based on progress in NAFTA talks, a White House official said on Wednesday night.

The move, first reported by the Washington Post, followed comments earlier in the day by a White House spokeswoman that the impending tariffs could exclude Canada, Mexico and a clutch of other countries "based on national security."

Trump was expected to sign a presidential proclamation to establish the tariffs during a ceremony on Thursday, but a White House official said later it could slide into Friday because documents had to be cleared through a legal process.

The president has faced mounting opposition to the tariffs from prominent congressional Republicans and business officials worried about their potential impact on the economy.

The tariffs would impose duties of 25 percent on steel and 10 percent on aluminium to counter cheap imports, especially from China, that the president says undermine US industry and jobs.

A senior US official said the measures would take effect about two weeks after Mr Trump signs the proclamation.

White House spokeswoman Sarah Sanders told a regular media briefing on Wednesday: "We expect that the president will sign something by the end of the week and there are potential carve-outs for Mexico and Canada based on national security, and possibly other countries as well based on that process.

"It will be country by country, and it will be based on national security," she said.

Sanders said Trump was considering several candidates to fill Mr Cohn’s position, while Mr Navarro said he was not short-listed for the job.

China warns against trade war

In his first tweet on Wednesday, the Republican president showed no sign of backing away from the tariffs, saying the United States had lost more than 55,000 factories and 6 million manufacturing jobs and let its trade deficit soar since the 1989-1993 administration of President George H.W. Bush.

Later, his tweets turned to trade with China, demanding that Beijing lay out plans for reducing its trade surplus with the United States by $1 billion, which appeared to have been raised during a meeting with a top Chinese official last week.

"China has been asked to develop a plan for the year of a One Billion Dollar reduction in their massive Trade Deficit with the United States," Mr Trump tweeted, without saying where the message had been conveyed.

China ran a record goods trade surplus with the United States last year of $375.2 billion

Chinese Foreign Minister Wang Yi said the country would take a necessary response in the event of a trade war with the United States, while warning that such a war would only harm all sides.

Mr Wang, speaking on the sidelines of an annual meeting of China’s parliament, said China and the United States did not have to be rivals, and history showed that trade wars were not the correct way to resolve problems.

"Especially given today’s globalisation, choosing a trade war is a mistaken prescription. The outcome will only be harmful," Wang said.

"China would have to make a justified and necessary response," he said.

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Wang said China had a long way to go on its path of modernisation, and that it "will not and need not displace the United States".