In a conference call with reporters on Thursday, officials at Citizenship and Immigration Services portrayed the change as a simple bureaucratic update.

To give greater flexibility to active service members forced by the country to travel, the agency previously allowed a child born abroad to military personnel to be granted the same citizenship privileges as one “residing” in the United States. That automatic conferral is being withdrawn, so such parents will have to rely on another area of immigration law for their child to obtain citizenship. The parent who is a United States citizen serving abroad must prove he or she lived in the country for five years, including a mandatory two years after the parent turned 14 years old. Time spent abroad serving in the military will count toward the five-year requirement.

An official for Citizenship and Immigration Services said the agency felt the previous policy was sound, “but it’s not what the law says.”

“If I could change the law to our interpretation, I would happily do so, but the fact of the matter is it’s just not what the law says,” the official said.

The policy change will also make it more difficult for parents to obtain citizenship for children they adopt abroad, said Irene Steffas, a lawyer who specializes in immigration and adoption. Some parents will need to travel back to the United States to fulfill the five-year residency requirement needed to obtain citizenship for their children, Ms. Steffas said.

Some advocates were skeptical that the impact would be as limited as the administration maintained.

“If they really think this only affects 25 people, why have they invested the time and resources to do it? It’s only because it’s one more step in their ongoing policy to restrict legal immigration,” Mr. Lester said. “But when you start doing it by going after the babies of active-duty military personnel, at some point somebody’s got to say this isn’t the way our system is supposed to work.”

Homeland security officials sought to clarify that the policy change had nothing to do with speculation that the administration would try to revoke birthright citizenship for certain families. Mr. Trump said just last week he was “seriously” considering ending the process by which babies born in the country automatically become citizens, which would be immediately challenged in court as a violation of the Constitution.