"If somebody tries to impose a trade war upon us, we will fight," Chinese Ambassador to the US Cui Tiankai said after US President Donald Trump on Thursday signed off on a plan to impose tariffs on imports from China and restrict Chinese foreign direct investment.

"We will do whatever we can to defend the legitimate interests," Cui said in a video posted on the embassy Facebook account. "Let me assure those people who intend to fight a trade war. We will certainly fight back. We will retaliate. If people want to play tough, we will play tough with them and see who will last longer."

The US memorandum directs Trade Representative Robert Lighthizer and Treasury Secretary Steven Mnuchin to respectively come up with specific plans on the tariffs and on Chinese FDI in the US.

The memo is a result of a Section 301 investigation under the US Trade Act of 1974 into China's laws, policies, practices or actions related to technology transfer, intellectual property and innovation.

While the memo posted on the White House website did not give a specific amount of the tariffs, Trump said on Thursday "it could be about $60 billion".

Also, unlike executive orders that take legal precedence and cannot be changed by a memorandum, a US presidential memorandum can be amended or rescinded by executive orders or another memorandum.

Trump said the two countries are in the midst of a large negotiation.

"We'll see where it takes us. But in the meantime, we're sending a Section 301 action," Trump said at the signing ceremony.

The move has drawn firm opposition from US businesses and lawmakers.

Lighthizer was grilled on the economic impact on US states at a Senate hearing on Thursday and at a similar session at the House of Representatives on Wednesday.

The US Chamber of Commerce and major US retailers earlier sent a letter and statement to Trump urging he not proceed with the broad tariffs on China.

"What avenues are we going to take to protect the $7 billion export agriculture market from my state from being retaliated against in this trade war that the president is basically embracing?" Maria Cantwell, a Democratic senator from Washington, asked Lighthizer.

"It's not something I think that in an economy like in the state of Washington, which is so trade dependent, is ready to embrace," she said. "They look at this 1980s view as a very retro policy: 'Let's start a trade war.' We'd like something more sophisticated."

China will "take all necessary measures" to resolutely protect its legitimate rights and interests that might be hampered by the possible US initiation of a Section 301 investigation into the country, the Ministry of Commerce said on Thursday.