US President Donald Trump's administration has ordered embassies to stop scheduling appointments for student visas as it prepares to expand social media vetting of such applicants.

In a copy of a memo sent to diplomatic posts, Secretary of State Marco Rubio said the pause would last "until further guidance is issued".

The message said social media vetting would be stepped up for student and foreign exchange visas, which would have "significant implications" for embassies and consulates.

It comes amid Trump's feud with some of America's most elite colleges, which he believes are too left-wing. He says some of them have enabled antisemitism on campus and uphold discriminatory admissions policies.

The state department memo, viewed by the BBC's US partner CBS News, directed US embassies on Tuesday to remove any unfilled appointments from their calendars for students seeking visas, but said those with appointments already scheduled could go ahead.

The diplomatic cable also said the state department was preparing for an "expansion of required social media screening and vetting" applicable to all student visa applications. It does not spell out what the vetting would look for.

Foreign students who want to study in the US are usually required to schedule interviews at an American embassy in their home country before approval.

Many institutions rely on foreign students for a significant chunk of their funding, as they often pay higher tuition fees.

When asked about student visas, state department spokesperson Tammy Bruce told reporters on Tuesday: "We take very seriously the process of vetting who it is that comes into the country, and we're going to continue to do that."

The Trump administration has frozen hundreds of millions of dollars in funding for universities and moved to deport students, while revoking thousands of visas for others. Many of these actions have been blocked by the courts.

The White House has accused some US universities of allowing pro-Palestinian activism on campus to be hijacked by antisemitism.

Colleges have accused the Trump administration of trying to infringe upon free speech rights.

Harvard University has been the focal point of President Donald Trump's ire. Last week, the Trump administration revoked Harvard's ability to enrol international students or host foreign researchers. A federal judge blocked the policy.

If the measure is allowed, it could deliver a devastating blow to the university, where more than a quarter of students are foreign.