CHANGSHA, China — The United States abruptly ordered China to close its consulate in Houston by Friday, striking at what U.S. officials described as an espionage hub for Beijing and sending relations between the world's two biggest political and economic powers to a new low.

China immediately promised to retaliate for the "unprecedented escalation," raising the specter of a tit-for-tat battle that could prompt Beijing to shutter at least one U.S. consulate soon and adopt more punitive measures later.

One potential target is the U.S. Consulate in Wuhan, which closed after the coronavirus epidemic began spreading across the city in January. The diplomatic missions in Wuhan and Houston are considered "sister" consulates.

The acrimony between Washington and Beijing has been growing for months, but it was not immediately clear why the United States decided to order the Houston consulate closed now.

The consulate was notified of the order early Tuesday, according to a former senior U.S. official with knowledge of the situation. U.S. agents monitoring the consulate observed personnel going to a Home Depot to buy barrels, which they used to burn documents, the former official said, speaking on the condition of anonymity because of the matter’s sensitivity.

“The Houston consulate is well known across the U.S. intelligence and law enforcement community for its malign activities,” the official said.

The out-of-a-spy-novel confrontation widens a conflict that already has spanned trade and technology, freedom of the press and religion, students and scientists, human rights principles and the race for a coronavirus vaccine.

Analysts on both sides say bilateral relations are at their worst since before 1979, when the United States formally recognized the People’s Republic of China.

After the news broke, a phrase that Mao Zedong, who established the communist-run People’s Republic of China, wrote in 1949 about the China-U.S. relationship began trending on the Chinese Internet: “Cast Away Illusions, Prepare for Struggle.”

“At this rate, I wouldn’t even be surprised if Trump decides to sever diplomatic relations with China someday,” said Chu Shulong, a professor in American politics and diplomacy at Tsinghua University, suggesting that this was part of President Trump’s reelection strategy.

Jessica Chen Weiss, a Cornell University expert on China’s foreign relations, said the move appeared calculated to drum up fears of China as an existential threat, rather than as an indictment of specific actions.

“Closing the consulate does not appear to be part of a coherent strategy to deter or compel China to alter its behavior,” Weiss said. “It looks more like a ‘shock and awe’ strategy to distract U.S. voters from the Trump administration’s disastrous response to the pandemic.”

The Trump administration ordered the consulate’s closure “to protect American intellectual property and Americans’ private information,” State Department spokeswoman Morgan Ortagus said Wednesday.

“The United States will not tolerate the PRC’s violations of our sovereignty and intimidation of our people, just as we have not tolerated the PRC’s unfair trade practices, theft of American jobs and other egregious behavior,” she said, using the abbreviation for China’s official name.

In a separate statement, the State Department accused China of having engaged “in massive illegal spying and influence operations,” interfering in “domestic politics,” coercing “our business leaders” and threatening “families of Chinese Americans residing in China, and more.”

The Chinese Embassy in Washington condemned what it called “an outrageous and unjustified move which sabotages China-U.S. relations.” The embassy said it has followed the rules laid out in the Vienna Convention on Diplomatic Relations and accused the United States of unlawfully opening diplomatic pouches and searching the contents.

“Because of the willful and reckless stigmatization and fanning up of hatred by the U.S. side, the Chinese Embassy in the U.S. has received threats to the safety and security of Chinese diplomatic missions and personnel more than once,” it said in a statement.

The Chinese government is suspected of using the Houston consulate as a hub for aggressive intelligence operations that U.S. officials said had gone too far, and its closure can be read as a rebuke and a warning to Beijing, said a U.S. official, who like others spoke on the condition of anonymity to discuss sensitive information.

Another U.S. official said that the People’s Liberation Army has been sending Chinese students to American universities, and that the Houston consulate is the epicenter of all the malign activities facilitated by China.

The closure came one day after the Justice Department unsealed indictments against two Chinese hackers, accusing them of stealing intellectual property from U.S. firms researching the novel coronavirus and other sensitive information.

The FBI is investigating clandestine Chinese activity in American research institutions, a law enforcement official said. Officers with the People’s Liberation Army have come to the United States masquerading as doctors and medical researchers and embedding with universities, research facilities and pharmaceutical companies, the official said.

On Monday, for example, federal authorities arrested a Chinese woman who had claimed to be a neurologist coming to California to conduct brain disease research at Stanford University. In reality, she is affiliated with the Chinese Air Force, according to the Justice Department.

Last month, U.S. authorities arrested a Chinese scientific researcher as he tried to fly out of Los Angeles. According to court documents, he said the head of his military university lab had ordered him to observe a lab at the University of California in San Francisco, and relay information so it could be replicated in China.

The United States expects foreign governments to conduct intelligence activities within its borders, but espionage usually follows a set of unspoken rules.

Sen. Marco Rubio (R-Fla.), the acting chair of the Senate Intelligence Committee, called the consulate a “central node of the Communist Party’s vast network of spies” and said that closing it “needed to happen.”

At a news conference in Copenhagen, Secretary of State Mike Pompeo said European intellectual property had also been stolen, “costing hundreds of thousands of jobs — good jobs for hard-working people all across Europe and America stolen by the Chinese Communist Party.”

“We are setting out clear expectations for how the Chinese Communist Party is going to behave, and when they don’t, we’re going to take actions that protect the American people, protect our security, our national security, and also protect our economy and jobs,” he added.

The first sign of the American order came when Houston NBC affiliate KPRC2 aired video showing people in the courtyard of the consulate apparently burning documents after 8 p.m. local time Tuesday. The consulate’s staff had been told it would be evicted from the building at 4 p.m. Friday, the Houston Chronicle reported.

In Beijing, the Foreign Ministry responded angrily to the order. The U.S. government “abruptly informed” China on Tuesday that it had to immediately close its consulate in Houston, Wang Wenbin, a Foreign Ministry spokesman, told reporters Wednesday.

“The U.S. has far more diplomatic missions and staff working in China. So if the U.S. is bent on going down this wrong path, we will resolutely respond,” he said.

In addition to its embassy in Beijing, the United States has consulates in Shenyang, Shanghai, Wuhan, Chengdu and Guangzhou. Analysts expect the Chinese government to respond by ordering one of them closed. The Wuhan consulate, evacuated in January as the coronavirus began spreading in the city, has not reopened, and the embassy and other consulates are operating with skeleton staffs, according to American officials.

Wang said the order came amid American attacks on China’s political system, harassment of Chinese diplomats and intimidation of Chinese students. He said the consulate had been getting prank phone calls and even received a bomb threat on Monday.

The United States and China have battled for supremacy since the start of the Trump administration, centered on trade and technology. Both sides have expelled journalists this year and slapped sanctions on each other’s officials.

But the hostilities have become much more serious with Trump’s efforts to blame the Chinese government for the coronavirus that emerged in Wuhan at the end of last year and retaliatory actions over journalists in the countries.

Two Chinese hackers are accused of a decade-long global theft operation, according to a Department of Justice indictment made public on July 21. (Reuters)

The Foreign Ministry on Wednesday renewed its travel warning for Chinese students in the United States, advising them to be aware of arbitrary interrogations, the confiscation of personal belongings and possible detentions.

Chu, the Tsinghua professor, agreed that there was potential for further tension.

“China could always evict a U.S. consulate in retaliation, and that could go on and on,” he said.

Morello, Nakashima and Harris reported from Washington. John Hudson in Copenhagen and Liu Yang and Lyric Li in Beijing contributed to this report.