Some of the world's most prominent aid groups and other nongovernmental organizations warned that their ability to brief U.S. and United Nations agencies on humanitarian crises will be hurt by a recent change in a 30-year-old visa program.

They also warned that the move could even hurt their ability to carry out much needed aid work in war zones, potentially having a damaging effect on lifesaving humanitarian operations.

They fear that the visa adjustments will mean that many of their staffers who spend time in some of the world's worst crisis-hit countries — such as Syria and Iraq — will be prevented from visiting the United States or face long delays in getting permission to enter the country.

The controversy centers on an amendment to the visa waiver program (VWP) that Congress recently rushed into an omnibus spending bill and President Barack Obama signed into law on Dec. 18.

The VWP allows citizens of 38 countries to travel to the U.S. without needing to obtain a visa. The legislation, which was introduced in response to the attacks in Paris and San Bernardino, California, introduced a number of changes, including requiring a visa for anyone who has visited Iraq, Syria, Iran or Sudan in the previous five years.

Lawmakers supporting the amendment are concerned that citizens of the 23 European Union countries that participate in the VWP could travel to the U.S. after receiving training in Syria from the Islamic State in Iraq and the Levant, or ISIL.

Aid workers say they were blindsided by the passing of the amendment. Alex Gray, the global humanitarian director at Relief International, said he didn’t expect the amendment to become law so quickly. “We heard discussion that this might be happening, but we thought it was just political gesturing,” he said.

NGOs are concerned that the provisions are too general and could affect the humanitarian work they do in those countries.

The amendment is “overly broad and contains discriminatory nationality provisions,” InterAction, an alliance of U.S.-based global NGOs, said in a letter to members of Congress. “For the NGO community, [the amendment] would limit the work of international humanitarians who are on the ground in Iraq, Sudan and Syria, providing lifesaving assistance to those who need it most.”

The U.N. lists Iraq and Syria as two of its four Level 3 emergencies — its classification for the largest and most severe humanitarian crises.

Gray said that because Iraq and Syria are Level 3 countries, the “bulk of the international humanitarian aid workers are actually traveling back and forth,” disqualifying them from entering the U.S. without a visa under the new law.

He is based in the United Kingdom and is one of those aid workers who benefit from the VWP. In the last few years, he has traveled many times to northern Iraq, Syria and Sudan.

Relief International, which provides water, sanitation and emergency health services to displaced people in countries surrounding Syria, has its staff from regional offices attend conferences in the U.S. quarterly to brief donors and policymakers. “We are already starting to talk about not having our global meetings in the U.S. anymore,” Gray said.

The American Civil Liberties Union has also warned that the amendment is too broad and called for exceptions to be made for those who have traveled to Syria, Iraq, Iran and Sudan for educational and professional purposes, including humanitarian aid workers.

"This includes weapons inspectors examining Iran nuclear facilities, social workers interviewing Kurdish refugees in Iraq, physicians treating patients in Darfur and human rights investigators documenting atrocities committed by ISIL," Joanne Lin, an ACLU legislative counsel, wrote in an article on the group’s website.

The changes have yet to be put into practice, according to the Department of Homeland Security (DHS), which is responsible for implementing the VWP provisions.

“At this time, no determination has been made as to how the waiver provision would be implemented,” DHS representative S.Y. Lee said in a statement. “We will announce any changes affecting travelers to the United States from visa waiver program countries as soon as that information is available.”

The DHS has not provided details on when that information will be available or indicated if exceptions will be made for aid workers.

“It concerns us because it impacts on our operations,” Gray said. “Until we have clear information in terms of how it’s being implemented, how can we plan in our procedures and operational decisions?”

Sara Margon, the Washington, D.C., director at Human Rights Watch (HRW), said she still “does not know how things are going to play out.”

“HRW staff who have traditionally benefited from the VWP and who have visited these countries of concern will, of course, follow the new procedures for visa applications, which could lead to delays or even denials. It all depends on how the legislation is implemented,” she said.

More than 20 million people visited the U.S. under the VWP in 2013, according to the DHS.

The amendment could affect Americans as well, NGOs warned. The VWP operates on the principle of reciprocity with other participating countries like France and Denmark. When the U.S. makes a change in its policy affecting their citizens, those nations will often make the same change regarding Americans.

“Requiring a visa to travel to those countries will add obstacles to their necessary work and may make it more difficult for Americans to manage employment as humanitarian workers,” InterAction said in its letter.