The new rule, published in the Federal Register and expected to take effect on Tuesday, also applies to children who have crossed the border alone.

It applies to anyone arriving at the US-Mexico border but seems to target Central Americans, who make up the largest number of immigrants.

There are some exceptions: If someone has been trafficked, if the country the migrant passed through did not sign one of the major international treaties that govern how refugees are managed (though most Western countries have signed them) or if an asylum seeker sought protection in a country but was denied, then a migrant could still apply for US asylum.

But the move by Donald Trump's administration is meant to essentially end asylum protections as they now are on the southern border, reversing decades of US policy on how refugees are treated.

It comes as the government continues to clamp down on migrants and as the treatment of those who made it to the country is heavily criticised as inhumane.

Attorney General William Barr said the US is "a generous country but is being completely overwhelmed" by the burdens associated with apprehending and processing hundreds of thousands of migrants at the southern border.

"This rule will decrease forum shopping by economic migrants and those who seek to exploit our asylum system to obtain entry to the United States," Mr Barr said in a statement.

The policy is almost certain to face a legal challenge.

The new rule will also apply to the initial asylum screening, known as a "credible fear" interview, at which migrants must prove they have credible fears of returning to their home country.

It applies to migrants who are arriving to the US, not those who are already in the country.

Trump administration officials said the changes are meant to close the gap between the initial asylum screening that most people pass and the final decision on asylum that most people do not win.

But immigrant rights groups, religious leaders and humanitarian groups have said the Republican administration's policies amount to a cruel and callous effort to keep immigrants out of the country.

Along with the administration's recent effort to send asylum seekers back over the border, Mr Trump has tried to deny asylum to anyone crossing the border illegally and restrict who can claim asylum, and the attorney general recently tried to keep thousands of asylum seekers detained while their cases play out.

Nearly all of those efforts have been blocked by courts.

Meanwhile, conditions have worsened for migrants who make it over the border seeking better lives.

Tens of thousands of Central American migrant families cross the border each month, many claiming asylum.

The numbers have increased despite Mr Trump's derisive rhetoric and hardline immigration policies.

Border facilities have been dangerously cramped and crowded well beyond capacity.

The Department of Homeland Security's watchdog found fetid, filthy conditions for many children.

And politicians who travelled there recently decried conditions.

Immigration courts are backlogged by more than 800,000 cases, meaning many people will not have their asylum claims heard for years despite more judges being hired.

Australian Associated Press