Donald Trump has suspended for an initial six months the entry of foreign students seeking to study or participate in exchange programmes at Harvard University.

The US president issued the proclamation on Wednesday, citing "national security" concerns and declaring it "detrimental" to US interests to continue allowing foreign students at the institution.

Within hours, Harvard responded by filing court documents asking a judge to block the "retaliatory" order from taking effect.

Trump's announcement is a further escalation of an ongoing legal row with one of the most prestigious US universities after Harvard refused to yield to a series of White House demands in April.

Wednesday's order comes after a judge blocked the Department of Homeland Security (DHS) from banning international students at Harvard in a ruling last week.

Trump's proclamation accused Harvard of developing "extensive entanglements" with foreign countries and continuing to "flout the civil rights of its students and faculty".

"Considering these facts, I have determined that it is necessary to restrict the entry of foreign nationals who seek to enter the United States solely or principally to participate in a course of study at Harvard University," he said.

The order also suspends visas for international students seeking exchange programmes and directs the secretary of state to consider revoking existing visas of students currently studying at the university.

The suspension can be extended beyond six months.

The White House said Harvard had failed to provide sufficient information to the DHS about "foreign students' known illegal or dangerous activities" and reported "deficient data on only three students".

Harvard issued a statement calling the order "yet another illegal retaliatory step taken by the administration in violation of Harvard's First Amendment rights" to free speech.

On Thursday, the university amended an existing lawsuit against the government, arguing that the latest move by Trump is "part of a concerted and escalating campaign of retaliation by the government in clear retribution for Harvard's exercising its First Amendment rights".

The lawsuit rejected claims that Trump is attempting to "safeguard national security", instead accusing him of "a government vendetta against Harvard".

University President Alan Garber also issued a statement saying that Harvard was developing contingency plans for international students, who make up around a quarter of the student population.

The world's wealthiest university has been embroiled in a legal battle with the Trump administration after it froze billions of dollars of federal funding and accused the institution of failing to root out antisemitism on campus.

Last month, DHS Secretary Kristi Noem revoked certification Harvard needed to enrol foreign students on campus, a move that was swiftly blocked by a judge.

Another federal judge upheld that decision last Thursday, saying she would issue a longer-term hold that would allow international students to continue their studies at Harvard while the legal battle plays out.

However, Wednesday's proclamation once again throws the futures of thousands of international students into limbo.

For the 2024-2025 school year, Harvard enrolled nearly 7,000 foreign students, who made up 27% of its population.

Last week, a Chinese Harvard student called for unity during the university's graduation ceremony, just days after Trump vowed to "aggressively" revoke visas for Chinese students.

In the past few months, the Trump administration has ramped up its crackdown on higher education in the US, accusing universities of failing to tackle antisemitism amid protests against the war in Gaza across campuses.

Earlier on Wednesday, the White House threatened to strip Columbia University of its accreditation over claims it violated the civil rights of its Jewish students.