Since it was activated for pre-screening last week, the app has been in such demand that it is telling applicants it has run out of appointments, according to Mexican officials and a dozen migrants, some of whom shared phone screenshots with Reuters.

To receive a U.S. appointment, migrants first must go to a border entry point in Mexico determined by the app. Some migrants told Reuters the app only had appointments far from where they currently are.

U.S. Customs and Border Protection did not immediately reply to requests for comment from Reuters about the app.