President Donald Trump’s executive order to halt the U.S. refugee resettlement program for four months could prevent thousands of people, who have already been through the 18 to 24-month vetting process, from having their chance to settle in the United States.

“This week alone, over 800 refugees were set to make America their new home, but instead find themselves barred from traveling to the U.S.,” the U.N. refugee agency said Monday in a statement.

Based on average monthly figures, this means that a 120-day suspension might prevent 20,000 refugees from being resettled.

“Refugees are anxious, confused and heartbroken at this suspension in what is already a lengthy process,” the statement said.

If the U.S. government stops processing or admitting people even for a week, “their exit visa expires or their medical expires, they have to go back and start all over,” Nina Zelic, director for refugee services for the Lutheran Immigration and Refugee Service, one of nine domestic resettlement agencies, told The Huffington Post last week. “The process is so synchronized that any stick in the wheel sort of throws it off pretty badly.”

The executive order also indefinitely bans Syrians from taking refuge in the United States and temporarily blocks those coming from Libya, Syria, Yemen, Iran, Sudan, Somalia and Iraq from entering the U.S. at all ― even if they have visas. Not to mention it reduces the refugee admittance quota for the 2017 fiscal year from 110,000 to 50,000.

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