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.
Harvard has responded by calling the order "retaliatory" and emphasised it would continue to protect its international students, according to Reuters news agency.
Trump's announcement is a further escalation of an ongoing legal row with one of the US's most prestigious universities after Harvard refused to yield to a series of White House demands in April.
