The United States has ordered China to close its consulate in Houston and accused it of ramping up spying operations, dramatically escalating diplomatic tensions between the feuding superpowers.

Wednesday's announcement infuriated Beijing, which vowed to retaliate as the two countries squabble over a slew of issues ranging from trade to the coronavirus pandemic -- and China's policies in Hong Kong, Xinjiang and the South China Sea.

"We have directed the closure of PRC Consulate General Houston in order to protect American intellectual property and Americans' private information," State Department spokeswoman Morgan Ortagus told reporters during a visit by US Secretary of State Mike Pompeo to Denmark.

She added that under the Vienna Convention, states "have a duty not to interfere in the internal affairs" of the receiving state, but there were no further details about the reasons behind the decision. In another statement, the State Department said China has engaged in massive spying and influence operations throughout the United States for years. "These activities have increased markedly in scale and scope over the past few years," it said.

Before the news emerged, firefighters and police were called Tuesday evening to the consulate building in Texas on reports that documents were being burned in trashcans the courtyard, according to local media.

The Twitter feed of the Houston police force said smoke was observed, but officers "were not granted access to enter the building". In Beijing, foreign ministry spokesman Wang Wenbin said China was told Tuesday that the consulate would have to close, calling it an "outrageous and unjustified move which will sabotage China-US relations".

"China urges the US to immediately withdraw its wrong decision, or China will definitely take a proper and necessary response," Wang said at a press briefing. "It is a political provocation unilaterally launched by the US side, which seriously violates international law... and the bilateral consular agreement between China and the US."

Diplomatic row

The Chinese Consulate in Houston was opened in 1979 -- the first in the year the US and the People's Republic of China established diplomatic relations, according to its website. The office covers eight southern US states -- including Texas and Florida -- and has nearly one million people in the area registered there.

There are five Chinese consulates in the US, as well as an embassy in Washington. State-run tabloid the Global Times launched a poll on Twitter in English asking people to vote for which US consulate in China should be closed in response, including the ones in Hong Kong, Guangzhou and Chengdu. Twitter is blocked in mainland China.

The US has an embassy in Beijing, plus five consulates in mainland China and one in Hong Kong. The order to close the Houston consulate came the same day that the US Justice Department indicted two Chinese nationals for seeking to steal coronavirus vaccine research.

The two men, who are believed to be in China, acted in some instances "for their own personal gain" and in others for the benefit of China's Ministry of State Security, Assistant Attorney General John Demers said at a news conference.

China rejected the allegations as "slander". The Chinese foreign ministry on Wednesday warned its citizens studying in the United States to be "on guard" as US law enforcement agencies "have stepped up arbitrary interrogations, harassment, confiscation of personal belongings and detention" of Chinese students.

More than 300,000 Chinese study in the US. President Donald Trump's administration has ramped up pressure on China on a wide range of issues, imposing sanctions on Chinese officials over policies in Tibet and Xinjiang.

The United States has led denunciations of the treatment of Muslim minorities in Xinjiang, a far west region where an estimated one million Uighurs and other ethnic groups are believed to have been held in re-education camps.

The US has also downgraded relations with Hong Kong after China implemented a new security law which Washington says is in violation of Beijing's promises of autonomy for the territory. In addition, last week Washington formally declared Beijing's pursuit of territory and resources in the South China Sea as illegal, explicitly backing the territorial claims of Southeast Asian countries against those of China.

Washington has also infuriated Beijing by banning telecom giant Huawei and seeking the extradition from Canada of top company executive Meng Wanzhou. Pompeo urged the "entire world" to stand up to China during a visit to Britain Tuesday.