Cracking down on migration has been a signature policy of President Donald Trump.

The Trump administration moved Monday to declare ineligible for asylum most migrants who cross the US southern border after passing through Mexico.

A new rule redefining asylum eligibility -- to take effect on Tuesday -- is the latest attempt to stem the flow of undocumented migrants into the country, and comes with the White House frustrated at Congress's failure to toughen immigration laws.

Cracking down on migration has been a signature policy of President Donald Trump.

"The United States is a generous country but is being completely overwhelmed by the burdens associated with apprehending and processing hundreds of thousands of aliens along the southern border," Attorney General Bill Barr said in a statement.

"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," Barr added.

Central American migrants targeted

The White House's new rule targets the recent stream of hundreds of thousands of migrants from Central America and other countries who have tried to cross into the United States from Mexico and request asylum.

These requests -- increasingly made by families saying they have fled endemic violence and poverty in their countries -- allow the applicants to remain in the United States and to move around freely while their cases are adjudicated, which can take two years.

Many disappear into the country and never appear in court for their hearings, immigration officials say.

The new rule declares ineligible for asylum any migrants who enter the United States from the southern border and who have not asked for asylum protection in any of the countries they crossed to get to the United States.

The rule includes exceptions for people who can demonstrate they are victims of torture or persecution.

But it was not clear whether the White House expects the migrants to seek asylum in the countries they first travel to -- Guatemala and Mexico mainly -- or simply apply for asylum in the United States while waiting in those countries.

- Legal challenge expected

The rule could quickly run into legal challenges that have repeatedly stymied the White House's efforts to slow the number of migrants entering the country without documentation.

The American Civil Liberties Union announced that it was preparing to sue to block the new rule.

"The Trump administration is trying to unilaterally reverse our country's legal and moral commitment to protect those fleeing danger. This new rule is patently unlawful and we will sue swiftly," said Lee Gelernt, deputy director of the ACLU Immigrants' Rights Project.

Democratic Representative Bennie Thompson, chairman of the House Committee on Homeland Security, called the new asylum rule "xenophobic" and "racist."

"It is unlawful and must be stopped by the courts. We need immigration policies that affirm the American values that the President is all too eager to trample on," said Thompson.

Migrant numbers hit 13-year high

The number of border-crossers detained by the US Border Patrol surged to a 13-year high of more than 144,000 in May before easing to 104,000 in June -- still up 142 percent from a year earlier.

Most are families from Guatemala, Honduras and El Salvador.

The large numbers of migrants has overwhelmed the capacity of US officials to house and care for them, with many, including children, being held in squalid conditions for weeks as they await processing.

Turmoil in Mexico, Guatemala

The US asylum policy has sparked political turmoil in both Mexico and Guatemala.

Neither wants to accept responsibility for transiting migrants or those rejected by the United States.

Mexican Foreign Minister Marcelo Ebrard said his country would not become a dumping ground for asylum seekers.

"Mexico does not agree with measures that limit asylum or refuge for those that fear for their lives in their country of origin," said Ebrard.

On Sunday, Guatemalan President Jimmy Morales cancelled a planned trip to Washington to discuss and possibly seal a deal making Guatemala a "safe third country" for asylum requests, after the idea was blocked by the country's constitutional court.