BEIJING — By threatening to raise taxes on Chinese imports, President Donald Trump is throwing down a challenge to Beijing: Agree to sweeping changes in China's government-dominated economic model — or suffer the consequences.

The unexpected ultimatum, delivered via tweets on Sunday and Monday, shook up financial markets that had expected the world's two biggest economies to resolve a yearlong standoff over trade, perhaps by the end of the week.

"It's a significant change in the president's tone," said Timothy Keeler, a partner at the law firm Mayer Brown and former chief of staff for the U.S. Trade Representative. "It certainly increases the possibility that you'll have no deal."

For weeks, Trump administration officials had been suggesting that the U.S. and Chinese negotiators were making steady progress. A Chinese delegation is due to resume talks Wednesday in Washington.

Suddenly on Sunday, Trump said he had lost patience: "The Trade Deal with China continues, but too slowly, as they attempt to renegotiate. No!" he tweeted.

The president vowed to raise tariffs on $200 billion in Chinese imports from 10% to 25% on Friday. And he said he planned "shortly" to slap 25% tariffs on another $325 billion in Chinese products, covering everything China ships to the United States.

Michael Pillsbury, director of the Hudson Institute's Center on Chinese Strategy and an adviser to the Trump White House, said the president's tweets suggest frustration that Chinese leaders "are trying to take back concessions they already made."

The two countries are engaged in high-stakes commercial combat over China's aggressive push to establish Chinese companies as world leaders in cutting-edge fields such as robotics and electric vehicles.

The United States accuses Beijing of predatory practices, including hacking into U.S. companies' computers to steal trade secrets, forcing foreign firms to hand over technology in exchange for access to the Chinese market and unfairly subsidizing Chinese firms at the expense of foreign competitors.

The Trump administration has imposed 10% tariffs on $200 billion in Chinese imports and 25% tariffs on another $50 billion. The Chinese have retaliated by targeting $110 billion in U.S. imports.

Global stock markets sank Monday on Trump's tweetstorm. But shares in the United States regained some of the lost ground on news that Chinese officials were planning to go ahead with this week's meetings in Washington. Still, the Chinese government did not provide details on exactly when talks would resume and who would be on China's negotiating team.

The lack of details suggested that Beijing is wrestling with an internal conflict: It is eager to end a trade fight that has battered Chinese exporters, but it doesn't want to look like it's bowing to the Trump administration's demands for far-reaching concessions.

Trump's threat makes going ahead with talks "very difficult politically" for President Xi Jinping's government, said Jake Parker, vice president of the U.S.-China Business Council. He said the Chinese public might "view this as a capitulation" if Beijing reached an agreement before Trump's Friday deadline.

The conflict is testing how far Beijing is willing to go in changing a state-led economic model it sees as the path to prosperity and global influence — and how much power Washington will have to enforce any agreement.

Beijing is willing to change industrial plans that provoke foreign opposition but wants to preserve the ruling Communist Party's dominant role in directing economic development, said Willy Lam, a politics specialist at the Chinese University of Hong Kong.

Chinese officials have said they are willing to let foreign companies participate in plans that call for government-led creation of global competitors in robotics and other technologies. But they have yet to release details, and it is unclear whether the concessions will satisfy Trump.