President Donald Trump has signed an executive order barring international students from entering the US to attend Harvard University, citing national security concerns. The order, effective immediately, suspends entry under Harvard-hosted student and exchange visitor visas.

According to Trump, Harvard’s conduct has rendered it an unsuitable destination for foreign students and researchers. The order alleges that Harvard failed to comply with Department of Homeland Security requests for information regarding alleged criminal and disciplinary activity involving international students.

“This is yet another illegal retaliatory step taken by the Administration in violation of Harvard’s First Amendment rights,” Harvard officials said in a statement. “Harvard will continue to protect its international students.”

The suspension is set to last six months, with the potential for extension. The order also places current international students and researchers at Harvard under review, directing the Department of State to consider revoking existing visas on a case-by-case basis.

Trump’s proclamation comes less than three weeks after the DHS attempted to strip Harvard of its certification to enroll international students through the Student and Exchange Visitor Program. A federal judge temporarily blocked the DHS action, allowing the University to continue enrolling international students.

Attorney General Pam Bondi defended the President’s proclamation, saying, “Admission to the United States to study at an ‘elite’ American university is a privilege, not a right.” Rep. Pramila Jayapal, D-Wash., criticized the measure, calling it “ridiculous” and a “thinly veiled revenge ploy” in Trump’s personal feud with Harvard.

The order provides a loophole to allow students whose entry would “benefit the national interest,” as determined by federal officials. For foreign students already at Harvard, Secretary of State Marco Rubio will determine if visas should be revoked.

The Trump administration’s efforts have created an environment of “profound fear, concern, and confusion” among international students, Harvard immigration services director Maureen Martin said in a court filing. Countless international students have asked about transferring from the university.

The dispute has been building for months after the Trump administration demanded a series of policy and governance changes at Harvard, calling it a hotbed of liberalism and accusing it of tolerating anti-Jewish harassment. Harvard defied the demands, saying they encroached on the university’s autonomy and represented a threat to the freedom of all US universities.