WASHINGTON (AP) -- President Donald Trump said Friday that he will no longer be "Mr. NICE GUY" with China on trade, declaring in a social media post that the country had broken an agreement with the United States.

Hours later, Trump said in the Oval Office that he will speak with Chinese President Xi Jinping and "hopefully we'll work that out," while still insisting China had violated the agreement.

What deal Trump was referring to was not clear. But the rhetoric was a sharp break from recent optimism when he lowered his 145% tariffs on Chinese goods to 30% for 90 days to allow for talks. China also reduced its taxes on U.S. goods from 125% to 10%.

"The bad news is that China, perhaps not surprisingly to some, HAS TOTALLY VIOLATED ITS AGREEMENT WITH US," Trump posted. "So much for being Mr. NICE GUY!"

Trump said the tariff reduction had "quickly stabilized" the Chinese economy, though the decrease also brought a degree of relief to U.S. companies that said the previous rates had essentially blocked their ability to bring in Chinese goods and imperiled their businesses.

The comments reflect the tensions between the world's two largest economies, as Trump is eager to show that his tariffs can deliver meaningful results in the form of U.S. factory jobs and increased domestic investment. The Trump administration also stepped up the clash with China in other ways this week, announcing that it would start revoking visas for Chinese students studying in the U.S.

Trump's negotiating style has often toggled between extreme threats and grand claims of progress. His mercurial approach has taken the financial markets on a wild ride of sell-offs and rallies that have produced a general sense of uncertainty.

That has been compounded by a court ruling this week that Trump had overstepped his legal authority with broad "Liberation Day" tariffs in April as well as import taxes on China, Canada and Mexico tied to fentanyl smuggling earlier this year. A federal appeals court on Thursday allowed Trump to temporarily keep collecting the tariffs under an emergency powers law while he appeals the earlier decision.

The Chinese Embassy in Washington said Friday that the two sides "have maintained communication over their respective concerns in the economic and trade fields" since officials met in Geneva nearly three weeks ago.

But the embassy also said the Chinese government had "repeatedly raised concerns with the U.S. regarding its abuse of export control measures in the computer chip sector and other related practices."

Both countries are in a race to develop advanced technologies such as artificial intelligence, with Washington seeking to curb China's access to the most advanced computer chips.

"China once again urges the U.S. to immediately correct its erroneous actions, cease discriminatory restrictions against China and jointly uphold the consensus reached at the high-level talks in Geneva," the embassy said.

Sun Yun, director of the China program at the Washington-based think tank Stimson Center, said, "I think the Chinese are playing hard to get with the trade talks."

Lin Jian, spokesman for the Chinese foreign ministry, on Friday accused the U.S. of overstretching the concept of national security by politicizing trade issues. He called the acts by the U.S. "malicious attempts to block and suppress China."

"We firmly oppose that and will resolutely defend our legitimate rights and interests," Lin said.

U.S. Treasury Secretary Scott Bessent said in a Thursday interview on Fox News' "Special Report" that talks with China had stalled.

Given the complexity and magnitude of the negotiations, "this is going to require both leaders to weigh in with each other," Bessent said. "They have a very good relationship. And I am confident that the Chinese will come to the table when President Trump makes his preferences known."

U.S. Trade Representative Jamieson Greer said Friday on CNBC that China has not removed non-tariff barriers as agreed.

"We haven't seen the flow of some of those critical minerals as they were supposed to be doing," Greer said.

China in December announced export bans to the U.S. of critical minerals including gallium, germanium and antimony. It announced more export controls on rare earth minerals in April, in response to Trump's tariffs.