In addition to the economic value, international students are a critical source of academic and research talent for colleges and universities in the United States.

Of the 54,664 doctorate recipients in 2017, more than 16,000 were non-US citizens, representing nearly 30 percent of the total.

This is according to the 2017 Doctorate Recipients from US Universities report published by the National Science Foundation in December. In physics, 25 percent of doctorates went to Chinese students alone, according to the foundation.

Steven Chu, a Nobel Prize winner for physics and former US secretary of energy, said at a recent forum on US-China relations at Stanford University, "The concern is that it's harder and harder to get visas, even for those very brilliant postdoctorate students who are applying to the best universities in the US.

He said the validity of visas for such students has been reduced to one year, from three or four years. "I would never hire a postdoc for one year," said Chu, professor of physics and professor of molecular and cellular physiology at Stanford University.

Last year, the administration of US President Donald Trump was reportedly considering a proposal to bar all Chinese students due to "espionage" concerns. Although the proposal was later shelved, the government has moved to place some restrictions on such students.

These include reducing the validity of visas for students and scholars from China studying and conducting research in certain "sensitive" fields.

Gordon Chang, a Stanford professor of American history and director of the Center for East Asian Studies at the university, said: "At Stanford, we don't do classified research. Overall, I don't see any irregularity or illegality going on."

He said the scrutiny of Chinese students worried him when he thought about academic freedom, openness and integrity.