WASHINGTON, June 9 — The number of migrants caught illegally crossing the US-Mexico border dropped on Friday, a senior US border official told Reuters, saying it signaled a restrictive new Biden administration policy was deterring some illegal immigration.

US Border Patrol arrested around 3,100 people crossing illegally, down roughly 20 per cent from the days before, the official said, requesting anonymity to discuss preliminary figures.

"It's still too early to say this is a definitive trend," the official said. "But I think it is indicative of some possible early success."

Immigration has emerged as a top issue for Americans in the months before Nov. 5 elections that will decide control of the White House and Congress. President Joe Biden, a Democrat seeking another term, faces Republican Donald Trump - an immigration hardliner - in a rematch of the 2020 contest.

Biden took office in 2021 pledging to reverse many of Trump's restrictive immigration policies but has toughened his stance in the face of record migrant arrests at the border.

Biden implemented a sweeping policy on Wednesday that generally bars migrants who illegally cross the US-Mexico border from claiming asylum. The asylum ban has exceptions for unaccompanied minors, people who face serious medical or safety threats, and victims of trafficking.

The new policy aims to maximise the number of migrants placed in "expedited removal," a fast-track deportation process. Since Wednesday, more than 2,000 people per day were put in expedited removal, more than double the previous rate, the US official said.

Questions still remain about whether border crossings will stay low enough to process people quickly and whether US authorities have the capacity to meet their goals.

The American Civil Liberties Union has vowed to sue to halt the measure, which resembles Trump-era curbs on asylum. — Reuters