BEIJING - China warned students on Monday to think about the "risks" associated with attending college in the United States, an apparent sign that the authorities in Beijing are expanding the boundaries of the trade war to include educational exchanges.

The warning, issued Monday, comes as the Chinese government looks for ways to retaliate against the Trump administration for the tariffs it has imposed on $250 billion worth of goods from China, ranging from fish to tungsten.

There are tentative plans underway for President Donald Trump and his Chinese counterpart, Xi Jinping, to meet at the G-20 in Japan later this month to try to find a way out of the protracted trade war. But analysts say they will only meet if there has been substantial progress - and that there is no progress of any sort so far.

Speaking to reporters in Beijing Monday, Xu Yongji, an official from the Ministry of Education, said that the Trump administration and the U.S. Congress had "politicized some normal China-U.S. educational exchanges and cooperation activities."

"[They] are cracking down on them under the banner of 'China threat' and 'China infiltration,' and they are stigmatizing Confucius Institutes as a tool for China," she said, referring to the programs through which Beijing has sought to expand Chinese language and cultural studies into American universities.

"[They] are accusing Chinese students and scholars in the United States of launching 'nontraditional espionage' activities and causing trouble for no reason," Xu said, advising current and potential students to "strengthen" their risk assessments before deciding to study in the United States.

Chinese students have made up an increasingly large proportion of international students in the United States in recent years, numbering more than 370,000 in the last academic year - or almost one-third of all international students. They have become a valuable source of income for many colleges.

But the rate at which they are being rejected for visas is concerning the authorities here.

In 2018, 331 of the 10,313 students who applied to study to the U.S. on Chinese government scholarships were rejected, according to government figures. That amounted to a rejection rate of 3.2 percent. But in the first three months of this year, 182 of the 1,353 students who applied - or 13.5 percent - were unable to go due to visa problems, the China Scholarship Council statistics showed.

Furthermore, officials said that visas are taking longer to issue and are being issued for shorter periods.

This comes at the same time as the United States has revoked the 10-year visas of some Chinese scholars dealing with relations with the United States.

"These kinds of behaviors have already hurt the dignity of Chinese students studying in the United States and have seriously hurt the feelings of the Chinese people," Xu said. "This American behavior is causing a cold snap in China-U. S. educational exchanges and cooperation."

In recent days, the Chinese Internet has lit up with news that Chinese citizens will now be required to submit information about their social media accounts when applying for a visa for the United States. This includes sharing usernames for Chinese services including WeChat, Weibo and Youku, the visa department of China CYTS Tours wrote on its online accounts.

The Chinese Embassy in the United States issued a notice on its website on Sunday about the new requirement, reminding citizens to "truthfully provide the application materials."

There is no quick end in sight in the trade war, which began when Trump vowed to close the United States' $400-odd billion trade deficit with China. He has imposed tariffs of 25 percent on $250 billion in Chinese imports and has threatened to add duties to the remaining $300 billion in goods that America imports from China.

The administration has also put Huawei, the Chinese telecom giant, on a blacklist that effectively bars U.S. companies from supplying it with computer chips, software and other components without government approval.

Beijing responded by imposing tariffs on $60 billion worth of U.S. products, but has run out of American goods to tax. That has led it take other reciprocal actions, including announcing a plan to establish a blacklist of "unreliable" foreign companies and organizations, effectively forcing companies around the world to choose whether they would side with Beijing or Washington.

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The Washington Post's Liu Yang contributed reporting.