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Among the many other screening methods, the United States has now begun asking the would-be visitors applying for visas, to provide their social media identities.

A State Department official, under condition of anonymity, told AFP that the new security procedures came into effect on May 25 for travellers deemed to present a risk.

Civil liberties advocates fear that the travellers who pose no terrorist threat may be condemned for expressing their political and religious opinions on social media. In a March 6 memorandum, Trump had vowed to tighten controls on who can enter the United States, based on their online behaviour.

According to the US official, consular officers can demand extra information from applicants they deem to require "more rigorous national security vetting."

"Such visa applicants will be asked to provide additional information, including their social media handles, prior passport numbers, additional information about family members, and a longer history of past travel, employment, and contact information," she said.

However, these changes will "affect only a fraction of one percent of the more than 13 million annual visa applicants worldwide," she added. There is no suggestion that travellers will have to surrender passwords of their accounts.

But during the search, if, for example, they have posted suspicious content on Facebook or any other social media platforms, they may have to face additional questions. The tougher visa rules also come after the US administration banned travellers in the Muslim world from using laptops on passenger flights.

This came to light after intelligence agencies reportedly found evidence that Islamic extremists perfected the technology to hide a viable bomb in a working computer.