In the face of prolonged trade talks with China, President Trump announced, on Thursday, that he is moving to impose an additional 10% tariff on the remaining $300 billion in Chinese imports.

The new tariffs are due to go into effect on September 1.

A U.S. delegation traveled to Shanghai this week to continue detailed trade negotiations. President Trump has characterized the meetings as “constructive.” However, the president wants China to make good on its promise to increase its purchase of American agricultural products, a condition that both sides had agreed to.

Trump made Thursday’s announcement on Twitter, stating, “We thought we had a deal with China three months ago, but sadly, China decided to re-negotiate the deal prior to signing. More recently, China agreed to buy agricultural product from the U.S. in large quantities, but did not do so.”

“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!” the president continued.

“We look forward to continuing our positive dialogue with China on a comprehensive Trade Deal, and feel that the future between our two countries will be a very bright one!”

China’s leadership has not yet responded to Trump’s announcement of new tariffs. China had retaliated against U.S. tariffs by imposing tariffs of its own on $60 billion in U.S. imports to China, effective June 1. Those tariffs primarily targeted U.S. agricultural exports.

The U.S. stock market responded negatively to President Trump’s announcement, dropping nearly 300 points shortly before Thursday’s market close.