Washington DC - All 55 million foreigners who hold visas for the US are subject to continuous review, the State Department warned Thursday as President Donald Trump ramps up his crackdown on visas and immigration.

"The Department's continuous vetting includes all of the more than 55 million foreigners who currently hold valid US visas," a State Department official said.

"The State Department revokes visas any time there are indications of a potential ineligibility, which includes things like any indicators of overstays, criminal activity, threats to public safety, engaging in any form of terrorist activity, or providing support to a terrorist organization."

The official did not say that all 55 million visas were under active review, but made clear that the Trump administration considered all of them fair game.

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The official, speaking on customary condition of anonymity, said that the Trump administration was stepping up scrutiny in particular for students.

"We're reviewing all student visas," the official said, saying the State Department was "constantly monitoring what people have said" on social media, which visa applicants are now required to show.

Secretary of State Marco Rubio has proudly targeted pro-Palestinian protesters, using an obscure law that allows him to rescind visas for people deemed to counter US foreign policy interests.

The State Department earlier said it has revoked 6,000 visas since Rubio took office in January with Trump.

It marks four times as many student visas as President Joe Biden's administration revoked in the same period the previous year, according to the State Department.