President Donald Trump lashed out at China for what he said is its unwillingness to buy American agricultural products and said it continues to “rip off” the U.S., just as the two nations resumed negotiations in Shanghai following a three-month breakup.

“China is doing very badly, worst year in 27 -- was supposed to start buying our agricultural product now -- no signs that they are doing so,” Trump said Tuesday on Twitter. “That is the problem with China, they just don’t come through.”

Departing the White House later for Jamestown, Virginia, Trump told reporters “we’re either going to make a great deal or we’re not going to make a deal at all.”

He said Beijing is willing to make concessions in trade talks but he’s not sure if he will accept them and that the decision on reaching a deal is up to him, not his Chinese counterparts.

U.S. stocks fell a second day amid a mixed bag of corporate results and growing speculation a trade deal with China remains elusive. The S&P 500 Index slumped, with overnight trading reaching lows after Trump criticized China just as his negotiators started talks in Shanghai. The price of soybeans and cotton also went lower.

Trump said that the U.S. has “all the cards” and warned that if he’s reelected in 2020 China faces a much tougher deal. He said “they always change the deal in the end to their benefit,” adding that “they should probably wait out our Election to see if we get one of the Democrat stiffs like Sleepy Joe." That’s a reference to Democratic presidential candidate, former Vice President Joe Biden.

Financial markets are on edge about the fate of talks between the world’s largest economies and a two-day Federal Reserve meeting that starts Tuesday. The U.S. central bank is expected to lower interest rates for the first time in more than a decade, partly because trade war concerns are slowing business investment.

U.S. delegates including Treasury Secretary Steven Mnuchin and Trade Representative Robert Lighthizer arrived in Shanghai to begin another round of talks with their Chinese counterparts. The Americans are set to attend a dinner at the Fairmont Peace Hotel on Tuesday evening, and talks are scheduled to pick up again on Wednesday in China’s bustling port city.

Expectations for a breakthrough in the trade talks remain low. The two sides are further apart than they were three months ago, when negotiations broke down and each side blamed the other for derailing attempts to reach a deal. China is pushing for compromise in the talks, with state media underlining this week that the U.S. should meet it “halfway.”

It’s unclear what triggered Trump’s latest outburst. U.S. soybean exports to China slumped in the first half of the year to the lowest level in more than a decade, while pork sales in June slipped from a month earlier.

China has said the purchase of American farming products will be contingent on the U.S. following through on Trump’s pledge to allow American companies to resume sales to Huawei Technologies Co. Speaking to reporters at an event in Sao Paulo on Tuesday, Commerce Secretary Wilbur Ross said he’s received more than 50 applications for special licenses for companies do business with Huawei, and that he and Trump met with “supply chain” representatives last week.

The administration put the Chinese telecommunications giant on a trade blacklist in May, which means American firms need a special license issued by the Commerce Department to ship to the company.

--With assistance from Philip Glamann, Emma Dong and Charlie Zhu.