President Donald Trump's administration is seeking unprecedented control of the nation's universities, taking particular aim at foreign students by revoking visas and deporting some of those involved in protests against the war in Gaza.

A cable signed Tuesday by Secretary of State Marco Rubio and seen by AFP orders embassies and consulates not to allow "any additional student or exchange visa... appointment capacity until further guidance is issued."

The government plans to ramp up vetting of the social media profiles of international applicants to US universities, the cable said.

Rubio earlier rescinded hundreds of visas and the Trump administration has moved to bar Harvard University from admitting non-Americans.

China's foreign ministry spokeswoman Mao Ning on Wednesday said Beijing urged Washington to "safeguard the legitimate rights and interests of international students, including those from China."

Hundreds of thousands of Chinese students attend US universities, long viewed by many in China as beacons of academic freedom and rigour.

The sweeping US measures have resulted in foreign governments moving to snap up affected students, with Japan and Hong Kong urging local universities to take in more international applicants.

In Taiwan, a PhD student set to study at the University of California told AFP they were left "feeling uncertain" by the visa pause.

"I understand the process may be delayed but there is still some time before the semester begins in mid-August," said the 27-year-old student who did not want to be identified.

"All I can do now is wait and hope for the best."

- Protests at Harvard -

The suspension of visa processing came as Harvard students protested on Tuesday after the government said it intended to cancel all remaining financial contracts, Trump's latest attempt to force the institution to submit to unprecedented oversight.

Trump is furious at Harvard for rejecting his administration's push for oversight on admissions and hiring, amid the president's claims the school is a hotbed of anti-Semitism and "woke" liberal ideology.

A judge issued a restraining order pending a hearing on the matter scheduled for Thursday, the same day as the university's graduation ceremony for which thousands of students and their families had gathered in Cambridge, Massachusetts near Boston.

The White House, meanwhile, doubled down in its offensive, saying that public money should go to vocational schools that train electricians and plumbers.

"The president is more interested in giving that taxpayer money to trade schools and programs and state schools where they are promoting American values, but most importantly, educating the next generation based on skills that we need in our economy and our society," spokeswoman Karoline Leavitt said on Fox News Tuesday evening. "We need more of those in our country, and less LGBTQ graduate majors from Harvard University."

Some Harvard students were worried that the Trump administration's policies would make US universities less attractive to international students.

"I don't know if I'd pursue a PhD here, six years is a long time," said Jack, a history of medicine student from Britain who is graduating this week and gave one name.

Harvard itself has filed extensive legal challenges against Trump's measures, which legal experts say are likely to be overturned by the courts.

Separately, alumni plan to file a lawsuit against Trump on June 9, filmmaker Anurima Bhargava told a virtual meeting staged by Crimson Courage, a grassroots alumni group.

The cutting of Harvard's contracts -- estimated by US media to be worth $100 million -- would mark the slashing of business ties between the government and the country's oldest university.

In the last few weeks, the elite educational and research powerhouse has seen billions of dollars in federal grants frozen and millions of dollars of federal contracts torn up.

The university has sued both to block the revocation of its right to recruit and sponsor foreign students, 27 percent of its total roll, as well as to overturn the withdrawal of federal funding.

On Monday, Trump vowed he would prevail in the increasingly public struggle with Harvard, claiming that foreign students there include "radicalized lunatics, troublemakers."