The US State Department has began implementing its requirement that nearly all US visa applicants submit their social media usernames, previous email addresses and phone numbers as part of the application process.
The new requirement, which could affect up to 15 million would-be travelers to the US, is part of a broad expansion of enhanced screening under the Trump administration.
The State Department regulations say people will have to submit social media names and five years' worth of email addresses and phone numbers.
Certain diplomatic and official visa applicants will be exempt from the stringent new measures.
However, people traveling to the US to work or to study will have to hand over their information.
"We are constantly working to find mechanisms to improve our screening processes to protect US citizens, while supporting legitimate travel to the United States," the department reportedly said.
“National security is our top priority when adjudicating visa applications, and every prospective traveler and immigrant to the United States undergoes extensive security screening,” the department said in a statement to the AP.
“We are constantly working to find mechanisms to improve our screening processes to protect US. citizens, while supporting legitimate travel to the United States.”
The official said national security is the top priority and every would-be traveler or immigrant already faced screening, including requirements to list their travel history, family member information, and previous addresses.
Previously, only applicants who needed additional vetting - such as people who had been to parts of the world controlled by terrorist groups - would need to hand over this data.
But now applicants will have to give up their account names on a list of social media platforms, and also volunteer the details of their accounts on any sites not listed.
The Trump administration first proposed the rules in March 2018.
At the time, the American Civil Liberties Union - a civil rights group - said there is "no evidence that such social media monitoring is effective or fair", and said it would cause people to self-censor themselves online.
US President Donald Trump made cracking down on immigration a key plank of his election campaign in 2016.
He called for "extreme vetting" of immigrants before and during his time in office.
On Friday Mr Trump vowed to impose gradually rising tariffs on Mexico unless the country curbed illegal immigration at the US southern border.