US President Joe Biden tightened the rules for migrants entering the US illegally from Mexico in the middle of the election campaign.
The measures, which just entered force, allow the authorities to deport people who have entered the country irregularly, in some cases without processing their asylum applications.
"I've come here today to do what Republicans in Congress, refuse to do, take the necessary steps to secure our border," Biden said in Washington on Tuesday.
The UN refugee agency UNHCR criticized the regulation as an undermining of the basic right to asylum. US civil rights activists have announced they will take legal action.
The new regulation applies as soon as the average number of illegal border crossings from Mexico exceeds 2,500 per day in a week. It will be cancelled if the number then falls below 1,500 again.
US media reported, citing the authorities, that the current figure is over 4,000 per day. Since the start of the budget year in October, there have been around 1.5 million "irregular encounters" at the southern border - meaning cases in which people were detained, usually briefly - or deported directly.
That is a higher number of cases than at the same time in previous years. Numbers in December 2023 were higher than ever before in a single month.
The authorities can barely keep up with the processing of asylum applications and the US also lacks accommodation and other resources for the new arrivals.
The president went on to say that "asylum will still be available" to those who try to enter the US legally and vowed to "never demonize" immigrants.
"But if an individual chooses not to use our legal pathways, if they choose to come without permission and against the law, they'll be restricted from receiving asylum and staying in the United States."
He said the ban would "remain in place until the number of people trying to enter illegally is reduced to a level that our system can effectively manage."
Biden accused former president Donald Trump, who is running against him in the presidential election in November, of cynically torpedoing urgently-needed legislation in Congress in order to take advantage of it during the election campaign.
He said he would have prefered bipartisan cooperation to better staff and fund the relevant authorities through appropriate legislation, but insisted that the Republicans had left him "no other choice."
Those affected by the decree are to be deported at short notice if they do not explicitly request asylum. Those who do so will be subject to stricter checks by border officials.
Previously, most asylum seekers were generally allowed to stay in the country until their court date - which is often years in the future due to the overburdened authorities.
Exceptions to Biden's decree are to apply to unaccompanied children and people who are seriously ill, as well as victims of human trafficking.
Many Latin Americans pass through Mexico as they seek to reach the US, fleeing from violence, poverty and political chaos at home.
