Washington, Jan 21 (AP) A wide-ranging series of steps that US President Donald Trump has promised to take to beef up security at the southern border began taking effect soon after he was inaugurated on Monday, making good on his defining political promise to crack down on immigration.

Some of the moves revive policies from his first administration, including forcing asylum-seekers to wait in Mexico, cracking down on asylum access and finishing the border wall. But others will mark sweeping new strategies, like an effort to end automatic citizenship for anyone born in America, pulling the military into border security and ending use of a Joe Biden-era app used by nearly a million migrants to enter America.

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Actual execution of such a far-reaching immigration agenda, expected under several executive orders, is certain to face legal and logistical challenges.

And few details have been released so far.

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But in a concrete sign of how the changes were already playing out, migrants who had appointments to enter the US using the CBP One app saw them cancelled minutes after Trump was sworn in, while Mexico agreed to allow people seeking US asylum to remain south of the American border while their court cases play out.

"I will declare a national emergency at our southern border. All illegal entry will immediately be halted, and we will begin the process of returning millions and millions of criminal aliens back to the places in which they came," Trump said in his inauguration speech to thunderous applause.

CBP One app disappears

The online lottery system gave appointments to 1,450 people a day at eight border crossings to enter on "parole", which Joe Biden used more than any president.

It was a critical piece of the Biden administration's border strategy to create new immigration pathways while cracking down on people who enter illegally.

Supporters say it brought order to a chaotic border. Critics say it was magnet for more people to come.

By midday Monday, it was gone.

A notice on the Customs and Border Protection website emphasised the app was no longer being used. Migrants who had scored coveted appointments weeks ago found them cancelled.

That includes Melanie Mendoza, 21, and her boyfriend. She said they left Venezuela over a year ago, spending more than USD 4,000 and travelling for a month, including walking for three days.

"We don't know what we are going to do," she said in Tijuana, Mexico, just on the other side of the border from San Diego.

Mexico agrees to take back migrants

The Trump administration will reinstate its "Remain in Mexico" policy, which forced 70,000 asylum-seekers in his first term to wait there for hearings in US immigration court.

Mexico, a country integral to any American effort to limit illegal immigration, indicated Monday that it is prepared to receive asylum-seekers while emphasising that there should be an online application allowing them to schedule appointments at the US border.

Immigration advocates say the policy put migrants at extreme risk in northern Mexico, where they were easily recognisable to cartels, who kidnapped them and extorted their families for money.

"This is déjà vu of the darkest kind," Krish O'Mara Vignarajah, president and CEO of Global Refuge, said.

She said policies like "Remain in Mexico" have "exacerbated conditions at the border, stoked fear within US communities, and undermined our global humanitarian leadership role while doing little to address the root causes of migration".

Aiming to end constitutional right to birthright citizenship

Anyone born in the United States automatically becomes an American citizen, including children born to someone in the country illegally or in the US on a tourist or student visa. It's a right enshrined in the 14th Amendment, ratified in 1868 in the wake of the Civil War and assured citizenship for all, including Black people.

That effort is certain to face steep legal challenges, and a White House official previewing the executive orders on condition of anonymity provided no information on how Trump intends to carry it out.

Migrants fear promised mass deportations

The orders previewed were less specific about how Trump will fulfil his pledge of mass deportations of at least 11 million people already in the country illegally. One edict will equip immigration officers with "authorities needed" to enforce the law.

Trump and his aides have repeatedly said they would scrap Biden's deportation priorities, which focused on people with criminal records or national security threats, to include all people without legal status.

Erlinda, a single mother from El Salvador who arrived in 2013, signed over legal rights to her US-born children, aged 10 and 8, to Nora Sanidgo, who has volunteered to be guardian for more than 2,000 children in 15 years, including at least 30 since December.

"I am afraid for my children, that they will live the terror of not seeing their mother for a day, for a month, for a year," said Erlinda, 45, who asked to be identified by first name only due to fears of being detained.

Plans for deportation arrests appeared to be in flux after news leaked of an operation in Chicago this week. Trump's "border czar" Tom Homan said on Fox News on Sunday that Chicago was "not off the table, but we're reconsidering when and how we do it".

He said the leak raised concerns about officer safety.

Bigger military role in border security

Trump will order the government, with defence department assistance, to "finish" construction of the border wall, though the White House official didn't say how much territory that would cover.

Barriers span about 720 km, slightly more than one-third of the border. Many areas that aren't covered are in Texas, including inhospitable terrain where migrants rarely cross.

The official did not say how many troops Trump was planning to send, saying that would be up to the secretary of defence, or what their role would be when they get there.

Sending troops to the border is a strategy that Trump and Biden both have used before.

Historically, troops have been used to back up Border Patrol agents, who are responsible for securing the nearly 3,218-km border separating the US from Mexico and not in roles that put them in direct contact with migrants.

Critics have said that sending troops to the border sends the signal that migrants are a threat.

Cartels as foreign terrorist organisations

The Trump administration also intends to designate criminal cartels as foreign terrorist organizations and specifically aims to crack down on the Venezuelan gang Tren de Aragua and remove its members from the US.

The homegrown street gang was born in Venezuela but has become a menace even on American soil and exploded into the US presidential campaign amid kidnappings, extortion and other crimes throughout the Western Hemisphere tied to a mass exodus of Venezuelan migrants.

Pausing permission for refugees

He also intends to suspend refugee resettlement for four months, the official said. For decades, the program has allowed hundreds of thousands of people fleeing war and persecution around the world come to the United States.

Trump similarly suspended the refugee program at the beginning of his first term, and then after reinstating it, cut the numbers of refugees admitted into the country every year. Under Biden, the program was rebuilt to the point that last year about 100,000 refugees were resettled in America -- marking a three-decade high.

What else is Trump planning?

The incoming administration will also order an end to releasing migrants in the US while they await immigration court hearings, a practice known as "catch-and-release", but officials didn't say how they would pay for the enormous costs associated with detention.

Trump plans to "end asylum", presumably going beyond what Biden has done to severely restrict it. It is unclear what the incoming administration will do with people of nationalities whose countries don't take back their citizens, such as Nicaragua and Venezuela. (AP)

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