NEW YORK, Dec. 27 -- The U.S. government has started requesting foreign travelers to provide their social media accounts when they arrive in the country, a move designed to spot potential terrorist threats but raised questions over public privacy.

The new policy went into effect last week and applies only to travelers who enter the United States through the Electronic System for Travel Authorization (ESTA), a visa waiver application that many visitors are required to submit before travelling to the country.

The currently "optional" request asks applicants to "enter information associated with your online presence," and offers a drop-down menu that lists platforms including Facebook, Twitter, Youtube and Instagram, and a blank space for users to input their account names on those sites.

The draft proposal of the new policy was unveiled in June as part of a move to improve Washington's ability to spot suspicious individuals and avoid potential terrorist threats.

Although the U.S. Customs and Border Protection said that it would not prohibit entry to those who did not provide details of their online presence, the controversial policy has enraged human rights activists, who call it an invasion of privacy and urge the authorities to provide clear and transparent guidelines.