U.S. Secretary of State Rex Tillerson has directed U.S. diplomatic missions to identify "populations warranting increased scrutiny" and toughen screening for visa applicants in those groups, according to diplomatic cables seen by Reuters.

He has also ordered a "mandatory social media check" for all applicants who have ever been present in territory controlled by the Islamic State, in what two former U.S. officials said would be a broad, labor-intensive expansion of such screening. Social media screening is now done fairly rarely by consular officials, one of the former officials said.

Four cables, or memos, issued by Tillerson over the last two weeks provide insight into how the U.S. government is implementing what President Donald Trump has called "extreme vetting" of foreigners entering the United States, a major campaign promise. The cables also demonstrate the administrative and logistical hurdles the White House faces in executing its vision.

The memos, which have not been previously reported, provided instructions for implementing Trump's March 6 revised executive order temporarily barring visitors from six Muslim-majority countries and all refugees, as well as a simultaneous memorandum mandating enhanced visa screening.

The new rules do not apply to citizens of 38 countries — including most of Europe and longstanding allies like Australia, New Zealand, Japan and South Korea — who can be speedily admitted into the United States under the visa waiver program. That program does not cover citizens from any country in the Middle East or Africa.

Getting a Visa to visit the U.S. is a long and extensive process for much of the world. It can take months or even a year, and often ends with a denial, even for countries not included in Trump's revised travel ban, like India or Indonesia. The New York Times reported in an article all the steps of the typical process.

In 2016, the United States issued more than 10 million visas.