The United States has begun implementing changes to the US Visa Waiver Program for citizens of 38 foreign countries.

Starting from Thursday, members of those countries are required to obtain a visit ahead of their US visit if they have been to Iran, Iraq, Sudan, or Syria on, or after, March 1st, 2011.

Also, citizens of the 38 countries in the waiver program who also hold nationality in any of the four nations just mentioned are no longer eligible for the waiver program.

But the US Secretary of Homeland Security has said the new restrictions can be waived on a case-by-case basis for individuals who may have traveled to any of the four countries as journalists or humanitarian aid workers, for example.

The US decided to change the rules of the visa waiver program after the Paris attacks in November, in which 130 people were killed, citing fears that the program could be a weak point in US security.