A federal judge in Boston on Thursday temporarily blocked US President Donald Trump from barring US entry of foreign nationals seeking to study or participate in exchange programs at Harvard University.

Under a two-page temporary restraining order granted to Harvard, US District Judge Allison Burroughs enjoined Trump's proclamation from taking effect pending further litigation of the matter amid an escalating dispute between the Ivy League school and Republican president.

The judge ruled that Trump's directive prohibiting foreign nationals from entering the United States to study at Harvard for the next six months would cause "immediate and irreparable injury" before the courts have a chance to review the case.

Burroughs last month had blocked Trump from implementing a separate order prohibiting Harvard from enrolling international students, who make up more than a quarter of its student body. Harvard on Thursday amended its lawsuit to challenge the new directive, claiming Trump is violating Burroughs' decision.

"The Proclamation denies thousands of Harvard’s students the right to come to this country to pursue their education and follow their dreams, and it denies Harvard the right to teach them. Without its international students, Harvard is not Harvard," the school said in the filing.

Burroughs' order on Thursday also continued a separate temporary restraining order she issued on May 23 against the administration's restriction on international student enrollment at Harvard.

Earlier on Thursday, White House spokesperson Abigail Jackson called Harvard "a hotbed of anti-American, anti-Semitic, pro-terrorist agitators," claims that the school has previously denied.

"Harvard’s behaviour has jeopardized the integrity of the entire U.S. student and exchange visitor visa system and risks compromising national security. Now it must face the consequences of its actions," Jackson said in a statement.

Trump cited national security concerns as justification for barring international students from entering the US to pursue studies at the Cambridge, Massachusetts-based university.

Under Trump's proclamation, the suspension would initially be for six months but could be extended. Trump's order also directed the US State Department to consider revoking academic or exchange visas of any current Harvard students who meet his proclamation's criteria.

In Thursday's court filing, Harvard said Trump had violated federal law by failing to back up his claims about national security.

"The Proclamation does not deem the entry of an alien or class of aliens to be detrimental to the interests of the United States, because noncitizens who are impacted by the Proclamation can enter the United States — just so long as they go somewhere other than Harvard," the school said.

The Trump administration has launched a multifront attack on the nation's oldest and wealthiest university, freezing billions of dollars in grants and other funding and proposing to end its tax-exempt status, prompting a series of legal challenges.

Harvard argues the administration is retaliating against it for refusing to accede to demands to control the school's governance, curriculum, and the ideology of its faculty and students.

The university sued after Homeland Security Secretary Kristi Noem announced on May 22 that her department was immediately revoking Harvard's Student and Exchange Visitor Program certification, which allows it to enroll foreign students.

Noem's action was temporarily blocked almost immediately by Burroughs. On the eve of a hearing before her last week, the department changed course and said it would instead challenge Harvard's certification through a lengthier administrative process.

Nonetheless, Burroughs said she planned to issue a longer-term preliminary injunction at Harvard's urging, saying one was necessary to give some protection to Harvard's international students.

Wednesday's two-page directive from Trump said Harvard had "demonstrated a history of concerning foreign ties and radicalism," and had "extensive entanglements with foreign adversaries," including China.

It said Harvard had seen a "drastic rise in crime in recent years while failing to discipline at least some categories of conduct violations on campus," and had failed to provide sufficient information to the Homeland Security Department about foreign students' "known illegal or dangerous activities."

The school in Thursday's court filing said those claims were unsubstantiated.