A US State Department official on Thursday said that continuous vetting applies to “all of the more than 55 million foreigners who currently hold valid United States visas”, including those who have already been admitted to enter the country.

“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 organisation,” the official said.

In a written comment to The Associated Press, the official said that if a visa holder presents these indications, their visa will be revoked and they will be subject to deportation if they are already in the US.

US President Donald Trump has made cracking down on illegal immigration a focus of his presidency, boosting resources to secure the border and arresting people in the country illegally.

Earlier this month, the US government announced that it could require bonds of up to $15,000 for some tourist and business visas under a pilot programme to crack down on visitors who overstay their visas.

The programme gives US consular officers the discretion to impose bonds on visitors from countries with high rates of visa overstays, according to a Federal Register notice, which adds that bonds could also be applied to people coming from countries where screening and vetting information is deemed insufficient.

In July, US broadcaster CNN reported that the US would soon require international visitors to pay a “visa integrity fee” of at least $250, a new addition to existing visa application costs, according to a provision in the Trump administration’s domestic policy bill.

The fee is to be paid at the time visas are issued. Although there will be no fee waivers for the payment, the provision states that travellers who comply with the terms of their visa can have the fee reimbursed after their trip.

In June, United States consulates in Karachi and Lahore requested that all applicants for F, M or J nonimmigrant visas make their social media accounts public for vetting.

US consular officers are now required to conduct a “comprehensive and thorough vetting” of all student and exchange visitor applicants to identify those who “bear hostile attitudes toward our citizens, culture, government, institutions, or founding principles”, said a US diplomatic cable.