WASHINGTON - President Donald Trump lashed out in fury on Monday over immigration, an outburst triggered by images of a "caravan" of hundreds of Central American migrants headed toward the US border.

For the second straight day, Trump took to Twitter to attack Mexico for allowing the 1,500-strong group to march unimpeded toward the United States, their daily progress mapped out by US media.

He threatened to abrogate NAFTA -- the US-Mexico-Canada free trade pact -- and demanded Congress pass tougher immigration legislation and support his plan for a wall along the Mexican frontier.

He also declared he no longer supported a replacement for DACA, the program that had offered protection against deportation for about 700,000 undocumented immigrants, most of them Mexicans, who came to the US as children.

"As ridiculous as it sounds, the laws of our country do not easily allow us to send those crossing our Southern Border back where they came from. A whole big wasted procedure must take place. Mexico & Canada have tough immigration laws, whereas ours are an Obama joke. ACT CONGRESS," Trump tweeted in the evening, following similar comments in the morning and on Sunday.

The administration’s own statistics showed that arrests of undocumented migrants soared in the first year of Trump’s presidency, while illegal border crossings plummeted.

Data from Immigration and Customs Enforcement suggested, however, that processes to deport arrested illegal immigrants have become even more backed up than before.

"Honduras, Mexico and many other countries that the US is very generous to, sends many of their people to our country through our WEAK IMMIGRATION POLICIES. Caravans are heading here. Must pass tough laws and build the WALL.

Democrats allow open borders, drugs and crime!"

Since taking office more than a year ago, Trump has focused on cracking down on legal and undocumented immigration.

He has repeatedly tried to link immigration with crime but immigrant defenders say many immigrants are fleeing poverty and violence. Activists accuse authorities of rounding up longtime residents with families and jobs on minor infractions.

1,500 headed to border

"Mexico is making a fortune on NAFTA," Trump said earlier on Monday.

"With all of the money they make from the US, hopefully they will stop people from coming through their country and into ours, at least until Congress changes our immigration laws!"

Mexican President Enrique Pena Nieto demanded that the United States respectfully pursue the continuing NAFTA trade talks, despite disagreements over immigration.

"We have been conducting serious negotiations," he said, expressing hope that "this spirit of positivity, mutual respect and cordiality" will continue, while looking for areas "that really favor the development of the three countries."

Mexico’s Interior Minister Alfonso Navarrete Prida rejected Trump’s accusations that Mexico was not trying to control illegal immigration.

"Of course we will act, let me be clear, in strict compliance with our migration laws, without accepting pressure from any country or anyone," Navarrete said, referring to the caravan.

Organised each of the past five years by activists of Pueblo Sin Fronteras, or People Without Borders, the 1,500 migrants say they have fled El Salvador, Guatemala, Honduras or Nicaragua because of poverty and widespread violence from criminal gangs.

They set off on March 25 from the southern Mexican state of Chiapas for the US border, where many hope to apply for asylum.

On Monday, they were passing through the mountainous southern Mexican state of Oaxaca.

Trump appears to have become aware of them over the weekend from a report on Fox News, which he regularly watches.

On Sunday he said Mexico "must stop the big drug and people flows, or I will stop their cash cow, NAFTA," he tweeted.

He accused the migrants of seeking to benefit from DACA, but a leader of the caravan countered that Trump is using it as an excuse to stop any deal on Deferred Action for Childhood Arrivals (DACA).

The programme would provide those qualified — as many as 1.8 million people — a path to citizenship.

"This is just a pretense to lay the blame somewhere else. He’s just using our movement, the caravan, as a pretext," caravan organizer Rodrigo Abeja said.

Abeja said the Mexican government was in a difficult position with the caravan, given the growing publicity attached to its daily progress.

"The Mexican government is under a lot of pressure from the United States to show that it has control over its borders," he said.

"But it also can’t take direct action (such as deporting the migrants) because there is already a lot of public exposure." — AFP