Trump and Xi on Saturday agreed to the cease-fire in a trade war that has seen the flow of hundreds of billions of dollars worth of goods between the world's two largest economies disrupted by tariffs.
China's foreign ministry, the only government department that holds a daily briefing foreign media can attend, has repeatedly referred questions on details to the commerce ministry, which has yet to say anything.
In his latest Twitter storm, Trump said that his team of negotiators was still "seeing whether or not a REAL deal with China is actually possible", adding that China is "supposed to" start buying more USA agricultural products "immediately". "Unless extended, they will end 90 days from the date of our wonderful and very warm dinner with President Xi in Argentina", Mr Trump said on Twitter.
Mr. Trump's 10 percent tariff on $200 billion of Chinese goods was set to increase to 25 percent on January 1.
"We wouldn't be surprised if no agreement can be reached within 90 days, with the higher tariff rate coming into force only a few months later than originally planned".
Trump sowed more confusion as he opened the door to lengthier negotiations with China, suggesting that they could extend beyond a 90-day deadline to reach a deal to avoid a massive tariff increase.
Chinese officials have said they are "confident in implementing" trade commitments made to the United States "as soon as possible", without giving details.
Mr Trump said this week on Twitter that Beijing had agreed to "reduce and remove" tariffs the 40% tariffs it places on United States cars.
A day later, President Trump said on his Twitter that "the negotiations with China have already started" and that the deal will probably happen.
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Pompeo said in Brussels on Tuesday the U.S. could abandon some worldwide agreements to thwart bad actors like China, which he accused of cynically exploiting World Trade Organization rules.
"China will start with the implementation of the specific matters in which consensus has been reached, the sooner the better", it added, without providing more details. In a series of Tuesday morning tweets, the president showed optimism about reaching a "fair deal", but stressed that he is "a Tariff Man" if talks fail.
U.S. Trade Representative Robert Lighthizer is taking over as lead negotiator from Mnuchin, who will continue to be involved in deals.
"Ninety days is very little time to fix these perennial issues", said Bill Adams, senior economist at PNC.
"Ultimately, I believe, we will be making a deal - either now or into the future", he wrote.
Officials from the USA and a number of other major economies have often criticized China for its slow approach to negotiations and not following through on commitments.
"We are no longer in the same buoyant economic or markets environment that we enjoyed earlier this year when threats of tariffs against China were first made", she said.
Global financial markets tumbled on Tuesday as doubts over what could realistically be accomplished in the tight negotiating window added to concerns about fading global growth. The statement did not mention whether Trump discussed tariffs with the automakers.
A Chinese official told Reuters that officials were "waiting for the leaders to return" before publicizing details.
According to the White House, China has agreed to buy a "very substantial" amount of agricultural, energy, industrial and other products from the USA to reduce a yawning trade gap, as well as to negotiate over Washington's assertions that Beijing steals American technology.