CIUDAD JUAREZ, Mexico (AP) — The Biden administration on Thursday will begin denying asylum to migrants who arrive at the US-Mexico border without first applying online or seeking protection in a country they passed through, marking a fundamental shift in immigration policy as the US readies for the end of a key pandemic restriction.

The new rule represents a significant shift in the way the US deals with asylum seekers who have been showing up at the border in huge numbers in anticipation of this week’s end of the use of a restriction known as Title 42 that allows the government to quickly expel migrants to Mexico. US officials warned of difficult days ahead as the program tied to the COVID-19 pandemic expires this week.

The rule announced Wednesday is part of new measures meant to crack down on illegal border crossings while creating new legal pathways, including a plan to open 100 regional migration hubs across the Western Hemisphere, administration officials said. US officials have detailed steps they’ve taken to prepare for what many are expecting to be a substantial increase in migrants attempting to enter the US.

Homeland Security Secretary Alejandro Mayorkas warned that the days and weeks ahead have the “potential to be very difficult.”

“Our plan will deliver results but it will take time for those results to be fully realized,” he said.

In Ciudad Juarez, migrants continued to arrive this week in small groups by train or bus — and leave daily to surrender to the US authorities. Fran Tovar, a 30-year-old electrician from Venezuela who left two children behind to try to reach the US, was crossing on Wednesday.

“There is fear and anguish,” said Tovar, adding that he has spent three months in Juarez trying to get an appointment through an app the US has encouraged migrants to use to present themselves at a border entry point and seek admission.

Tovar was expelled from the US on his first attempt, then tried again 24 hours later. He was worried because after Title 42’s use expires, he could face tough consequences.

The COVID-19 pandemic-related restrictions have allowed border officials to quickly return people — and they did so 2.8 million times since March 2020. But after the restrictions expire Thursday, migrants caught crossing illegally will not be allowed to return for five years. They can face criminal prosecution if they do.

Even with the restrictions in effect, border crossings rose to all-time highs.