(CNN) President Donald Trump signed a presidential proclamation Friday morning that will bar migrants who cross into the US illegally through the southern border from seeking asylum.

Trump told reporters that he had "just signed" the proclamation as he departed the White House on Friday morning en route to Paris.

The proclamation put into effect a new rule the Trump administration entered into the federal registry on Friday that would ban migrants from applying for asylum outside of official ports of entry. The American Civil Liberties Union has already called the rule "illegal," and legal challenges are expected to follow.

The executive action is the latest the President has taken to clamp down on illegal immigration and to discourage the group of migrants now traveling through Mexico, many of whom want to seek asylum in the US, from making their way toward the country. Trump announced Thursday that he would make such a move.

The proclamation will go into operational effect Saturday morning, DHS and DOJ officials announced on a call with reporters Friday morning. The impact will be that most migrants who cross the southern border illegally will no longer be eligible for asylum claims in the United States for at least 90 days.

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