REUTERS/Kevin Lamarque

President Trump said Thursday he would expand tariffs to virtually all imports from China on September 1.

The announcement marked a significant escalation in a more than yearlong trade war between the largest economies.

Trump said China had not followed through with pledges to buy more American agricultural products and reduce the flow of fentanyl into the US.

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President Donald Trump said Thursday he would expand tariffs to virtually all imports from China on September 1, marking a significant escalation in a more than yearlong trade war between the largest economies.

Trump announced on Twitter the US would hit China with tariffs on roughly $300 billion worth of its products, a move he had previously said he would hold off on throughout negotiations. About $250 billion worth of Chinese goods already subject to a 25% duty.

Trump accused China of reneging on past commitments to the US days after high-level negotiations resumed between the two sides. He said it had not followed through with pledges to buy more American agricultural products and to reduce the flow of fentanyl into the US.

"Additionally, my friend President Xi said that he would stop the sale of Fentanyl to the United States – this never happened, and many Americans continue to die!" Trump said.

He added that the two sides would continue "positive dialogue" in the meantime. US Trade Representative Robert Lighthizer and Treasury Secretary Steven Mnuchin resumed talks with their Chinese counterparts in Shanghai this week, but those ended with little evidence of progress. The White House said Wednesday it expected negotiations would continue in Washington in early September.

The Trump administration has struggled to win concrete commitments from China on issues identified last year in its Section 301 investigation, which has also led to retaliatory tariffs on roughly $110 billion worth of American products.

Nearly a dozen rounds of trade trade talks fell apart in May after the US said China reversed on concessions in a draft agreement, prompting Trump to raise tariffs on a large swath of its products.

Trump has in recent months oscillated between expressing optimism toward a deal and threatening further escalation. On Tuesday he said China would have to make significant progress in negotiations with the US or risk "no deal at all."

Read more: Why Trump's attacks on the Fed have gotten more frequent and aggressive since he launched his reelection campaign

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