Washington (CNN) The Trump administration is making it more difficult for the children of some US service members and US government employees living abroad to automatically become US citizens, according to a policy alert released Wednesday by US Citizenship and Immigration Services.

The rule appears to primarily affect the children of naturalized US citizens serving in the armed forces who have not lived in the US for a required period of time, a relatively small number -- estimated to be approximately 100 annually, according to a Defense Department official.

It does not impact anyone born in the United States.

US citizenship can be acquired a few ways, including being born in the country. Children born abroad can acquire citizenship through their US citizen parents either at birth or before the age of 18.

While the latest policy guidance doesn't make anyone ineligible for citizenship, it appears to narrow how children abroad can gain citizenship. President Donald Trump has occasionally voiced his support for ending birthright citizenship and said last week he was "seriously" considering ending it, though it's unclear how he'd have the legal authority to do so. Acting USCIS Director Ken Cuccinelli said on Twitter that the new policy "does NOT impact birthright citizenship."

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