Washington DC - President Donald Trump ramped up his war on US universities Wednesday, banning visas for all foreign students coming to attend Harvard and threatening to strip Columbia of its academic accreditation.

Trump is pursuing an ideological purge of universities, claiming their international students pose a national security threat and that perpetuate liberal "bias."

A proclamation issued by the White House late Wednesday declared that the entrance of international students to begin a course at Harvard would be "suspended and limited" for six months, and that existing overseas enrollees could have their visas terminated.

"Harvard's conduct has rendered it an unsuitable destination for foreign students and researchers," said the order.

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"I'm trembling. This is outrageous," Karl Molden, a Harvard government and classics student from Austria, told AFP. "He is abusing his executive power to harm Harvard as much as he can."

"My god!" said another international student at Harvard, who declined to be named for fear of retribution, on learning of the executive order. "This is such a disgrace."

The announcement came after the Trump administration's earlier efforts to terminate Harvard's right to enroll and host foreign students were stalled by a judge.

The government already cut around $3.2 billion of federal grants and contracts benefiting Harvard and pledged to exclude the institution from any future federal funding.

Harvard has been at the forefront of Trump's campaign against top universities after it defied his calls to submit to unprecedented government oversight of its curriculum, staffing, student recruitment, and "viewpoint diversity."

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Trump has also singled out international students at Harvard, who in the 2024-2025 academic year accounted for 27% of total enrollment, and a major source of income.

"This is yet another illegal retaliatory step taken by the Administration in violation of Harvard's First Amendment rights," a university spokesperson said. "Harvard will continue to protect its international students."

The latest sweeping action against Harvard came as Trump's education secretary threatened Wednesday to strip Columbia University of its accreditation.

The Republican has targeted the school due to its students' prominent role in the movement to opposed Israel's US-backed destruction of Gaza.

Most institutions – including Columbia – have already caved to far-reaching demands from the Trump administration and brutally cracked down on their students and staff.

Withdrawing Columbia's accreditation would see it lose access to all federal funding – a very significant proportion of the university's income.

Students attending the university would also not be able to receive federal grants and loans towards tuition.

Following Wednesday's announcement, a Columbia spokesperson said the university "aware of the concerns" raised by the government with its accreditation body.

"We have addressed those concerns directly with Middle States," the spokesperson said, adding that "Columbia is deeply committed to combating antisemitism on our campus."

"We take this issue seriously and are continuing to work with the federal government to address it."