College administrators in the Boston area are sounding the alarm about long delays in processing visas for students and new graduates to take temporary jobs to complete training in their careers.

Jay Vyas, who directs the residency program at Massachusetts General Hospital, says international residents — from Canada to China — have seen long delays in obtaining Optional Practical Training or OPT visas.

“Without their visa being approved by the federal government, they are not allowed to move through the next stage of credentialing,” Vyas explained. “If you're not credentialed, you can't take care of patients, and that has an immediate effect on our hospital."

MGH is the largest teaching hospital of Harvard University, where more than 900 international students applied for visas this winter, but to date this summer fewer than 400 have been approved, administrators say. MGH has about 200 medical residents at a time.

“These residents have gone through an incredible selection process,” he said. “These are people who, in the future, are going to cure cancer and are going to make incredible impact on American medicine."

At one point this summer, about half of the 50 MIT students who applied for work visas were still waiting. But as of July 30, all but two had theirs approved.

Immigrant advocates say the delays aren't limited to selective colleges in the northeast.

"They're nationwide. This is a serious problem all around the country," said Ron Klasko, a leader of the American Immigration Lawyers Association.

Until this year, Klasko said, the average wait time for an OPT visa, which allows college students to get training related to their education, was about three months or less.

"In fact, there was a procedure in place that if it was not approved within three months, you could go to the local office and get it approved," Klasko said.

Last year, however, the Trump administration eliminated that rule, and now many cases take between five to seven months. The administration also unveiled a policy that makes it easier for international students to be charged with being in the country illegally, stoking fears of a kind of scientific McCarthyism taking hold.

"All of these applications are being delayed, and denial rates are increasing tremendously across the board,” Klasko said. “So between delays and denials the end result is legal immigration is being cut back.”

A United State Customs and Immigration Services official told WGBH News that the federal agency has recently experienced a surge in employment authorization requests. As a result, there's a "small backlog of cases that are pending beyond the standard 90-day processing time." The agency says it has implemented a plan to return to standard processing times soon.

On campuses, academics like William Kirby, who teaches China Studies at Harvard Business School, are skeptical. He says even established scholars are experiencing delays, not just students and new graduates.

"I certainly don't know of any smoking gun, but I've seen it in practice: Highly reputable Chinese academics having difficulty getting what would ordinarily be routine visas to come to the United States being slow-walked," Kirby said.

And if these delays continue, Kirby says a lot is at stake for American higher education — and the country.

“What's at stake is its tradition and reputation for openness, for being a place in which the best in the world compete for the best slots and the best universities,” he said.

Higher education is one of the few industries in this country that is still ranked number one, but academics worry continued visa slowdowns could undermine that ranking.