WASHINGTON/PIEDRAS NEGRAS, Mexico — US President Donald Trump on Monday kicked off his sweeping immigration crackdown, tasking the US military with aiding border security, issuing a broad ban on asylum and taking steps to restrict citizenship for children born on US soil.

Declaring illegal immigration a national emergency, Trump ordered the Pentagon to provide support for border wall construction, detention space, and migrant transportation, and empowered the Secretary of Defense to send troops to the border as needed.

Trump called for his administration to reinstate his "Remain in Mexico" program, which forced non-Mexican migrants to wait in Mexico for the resolution of their US cases.

Shortly after the inauguration, US border authorities said they had shut down outgoing president Joe Biden's CBP One entry program, which had allowed hundreds of thousands of migrants to enter the US legally by scheduling an appointment on an app. Existing appointments were canceled, leaving migrants stunned and unsure of what to do.

Trump, a Republican, recaptured the White House after promising to intensify border security and deport record numbers of migrants. He criticized Biden for high levels of illegal immigration during the latter's presidency, but as Biden toughened his policies last year and Mexico stepped up enforcement, the number of migrants caught crossing illegally fell dramatically.

Republicans say large-scale deportations are necessary after millions of immigrants crossed illegally during Biden's presidency. There were roughly 11 million immigrants in the US living illegally or with temporary status at the start of 2022, according to a US government estimate, a figure that some analysts now place at 13 million to 14 million.

"As commander-in-chief, I have no higher responsibility than to defend our country from threats and invasions, and that is exactly what I am going to do," Trump said in his inaugural address.

Trump's critics and immigrant advocates say mass deportations could disrupt businesses, split families and cost US taxpayers billions of dollars.

The American Civil Liberties Union, or ACLU, said in a federal court filing on Monday that Trump's decision to end the CBP One program removed the only avenue to asylum at the US-Mexico border, an opening salvo by the civil rights group to fight Trump's agenda in court.

People in the US have grown less welcoming toward immigrants without legal status since Trump's first presidency, but remain wary of harsh measures such as detention camps, a Reuters/Ipsos poll in December found.

In several Mexican border cities, migrants saw their appointments on Biden's CBP One app canceled just after Trump took office. Some 280,000 people had been logging into the app daily to secure an appointment as of Jan 7.

Migrants alarmed

Migrants waiting in Ciudad Juarez scrambled to find short-term rentals, buy bus tickets and call family members back home.

Denia Mendez, a Honduran sitting in the courtyard of a migrant shelter in Piedras Negras, across from Eagle Pass, Texas, opened her email inbox 30 minutes after Trump became president. She stared at an email for several minutes, reading it over and over, before her eyes welled up.

"They canceled my appointment," she said.

In his order focused on so-called "birthright citizenship", Trump called on US agencies to refuse to recognize the citizenship of US-born children unless at least one parent is a US citizen or a permanent resident.

His order prompted the swift filing of a lawsuit in federal court in New Hampshire by the ACLU and other groups, who argued that Trump's order violated the right for anyone born in the US to be considered a citizen enshrined in the Citizenship Clause of the US Constitution's 14th Amendment.

"Denying citizenship to US-born children is not only unconstitutional — it's also a reckless and ruthless repudiation of American values," Anthony Romero, the ACLU's executive director, said in a statement.

In other orders, Trump suspended US refugee resettlement for at least three months and ordered a review of security to see if travelers from certain nations should be subject to a travel ban.

Agencies Via Xinhua