Migrants caught illegally crossing the US-Mexico border could be denied the chance to claim asylum and quickly deported or turned back to Mexico under new restrictions announced on Tuesday, part of a sweeping enforcement effort by US President Joe Biden.

The new measures will take effect immediately and will have exceptions for unaccompanied children, people who face serious medical or safety threats and victims of trafficking, a senior official said on a call with reporters.

Biden, a Democrat, has toughened his approach to border security as immigration has emerged as a top issue for Americans in the run-up to November 5 elections where he will face/ Republican Donald Trump in a rematch of the 2020 contest.

The new asylum restrictions are not permanent, the US official told reporters. They are activated when the daily average of border arrests tops 2,500 over a week and will be paused when arrests drop below 1,500 per day, the official said.

The deterrent measures "will significantly increase consequences for those who cross the southern border unlawfully," the official said, requesting anonymity as a condition of the call.

Key operational questions about how the new measures will be implemented remained unclear, however, including how the administration would quickly deport migrants from far-away and uncooperative countries and how many non-Mexican migrants Mexico would accept under the new enforcement regime. (Reuters)