Washington, DC - US Secretary of State Marco Rubio on Tuesday ordered a suspension of student visa processing as President Donald Trump's administration ramps up vetting of their social media, according to an internal cable.

It is the latest move that takes aim at international students, a major source of revenue for US universities, after Rubio rescinded hundreds of visas and the Trump administration moved to bar Harvard University from admitting any non-Americans.

A cable signed by Rubio and seen by AFP orders embassies and consulates not to allow "any additional student or exchange visa... appointment capacity until further guidance is issued."

It said the State Department "plans to issue guidance on expanded social media vetting for all such applications."

Donald Trump Trump lashes out at "CRAZY" Putin and threatens Russia with "downfall" after massive Ukraine attack

The cable suggested that the suspension could be brief, telling embassies to receive new guidance in the "coming days," although US missions already frequently see major backlogs in processing applications.

State Department spokeswoman Tammy Bruce did not comment directly on the cable but said that "we take very seriously the process of vetting who it is that comes into the country."

"It's a goal, as stated by the president and Secretary Rubio, to make sure that people who are here understand what the law is, that they don't have any criminal intent, that they are going to be contributors to the experience here, however short or long their status," she said.

Asked if students seeking to study at US universities should expect visas to be ready before terms begin in the autumn, Bruce said only: "If you're going to be applying for a visa, follow the normal process, the normal steps, [and] expect to be looked at."

Rubio last week told a Senate hearing that he has revoked "thousands" of visas since Trump took office on January 20.

An obscure law allows the secretary of state to remove foreigners for activities deemed counter to US foreign policy interests.

The most visible targets have been students involved in activism over Gaza. Trump administration officials accused students of antisemitism, charges strenuously denied by a number of the people targeted.