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.”

Trump said later Tuesday that he had spoken recently with Chinese President Xi Jinping without elaborating on when or the substance of the discussion.

Trump 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 pared losses as Trump said he spoke with Xi, though the S&P 500 Index remained on track for a second straight decline. Stocks slid earlier in the day after Trump criticized China.

Trump said that the U.S. has “all the cards” and warned that if he’s re-elected 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 started 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.

After the president’s tweets, soybean futures extended a decline, falling as much as 0.4% in Chicago.