The United States on Thursday began enforcing new visa rules on some travelers who have visited or who have dual nationality with states considered seedbeds of terrorism, AFP reports.

The Department of Homeland Security said would-be visitors to the United States who have been to Iran, Iraq, Sudan or Syria since March 2011 will now always have to apply for a visa.

This will be the case even if the traveler is from a country in the U.S. visa waiver program — the 40 nations seen as friends of America whose citizens can visit freely.

In addition, citizens of visa waiver countries who hold dual Iranian, Iraqi, Sudanese or Syrian nationality will have to apply for a full visa before travelling to America.

The department said it had started to implement the new rules on Thursday, but there had already been reports of travelers falling afoul of the controversial regulations, according to AFP.

On Wednesday, the BBC reported that its journalist Rana Rahimpour, who has joint British and Iranian nationality, had been kept from boarding a U.S.-bound flight.

The State Department refused to comment on specific cases.

“We will carry out the law that Congress passed and the president signed,” a senior administration official told AFP, speaking on condition of anonymity.

“The Department of Homeland Security… is working closely with the Department of State and other partners to ensure that the new amendments…are appropriately implemented.”

Homeland Security said dual nationals and travelers who had visited the four targeted countries would still be eligible for visas if they apply for them properly.

But they will no longer be able to skip the visa process by registering with the Electronic System for Travel Authorizations (ESTA) like fellow waiver country citizens.

Members of allied forces who fought alongside American troops in Iraq will be exempted from the new rules, and aid workers and journalists may be exempted on a “case-by-case” basis.

The announcement of the new rules comes one day after Senate Democrats blocked consideration of a Republican bill that would curb the flow of Syrian and Iraqi refugees into the United States in order to prevent terrorists from slipping in.

The bill was brought up for discussion after no less than 24 states announced they would block the program to resettle Syrian migrants within their borders, following the Paris attacks in November, in which one of the terrorists snuck into Europe with a group of Syrian refugees.

President Barack Obama had warned he would veto the bill if it made it to his desk, and also told state officials across the country that states do not have legal authority to refuse to accept Syrian refugees.