Faced with mounting political pressure over the influx of immigrants at the US southern border, President Joe Biden on Tuesday signed an executive order, which would deport migrants who cross the border illegally without first examining their asylum claims when the daily number of arrivals exceeds 2,500.

The decree will take effect immediately as the limit has already been exceeded, said a senior official. The borders will only reopen when that number drops to 1,500. The president’s order is part of Sections 212(f) and 215(a) of the Immigration and Nationality Act which suspend the entry of non-citizens who illegally cross the southern border into the United States.

Senior officials said on Tuesday that “people who cross the southern border illegally or without authorization will not be selected for asylum, absent extremely compelling circumstances.”

Biden has hardened his stance as immigration has emerged as a top issue for Americans ahead of the Nov. 5 election.

The new asylum restrictions are not permanent, a US official said. They are activated when the daily average of border arrivals exceeds 2,500 in a week and will be lifted when arrivals fall below 1,500 per day.

But key operational questions about how the new measures would be implemented remained unclear, including how quickly the administration would deport immigrants from distant and uncooperative countries and how many non-Mexican immigrants Mexico would accept under the new enforcement regime .