WASHINGTON - US President Donald Trump on Wednesday ordered the National Guard to deploy to America’s southern border, ratcheting up pressure on Mexico and taking another step in his quest to clamp down on illegal immigration.

Trump’s latest border move came the same day as a caravan of Central American migrants -- whose trek across Mexico had infuriated the US president -- scrapped their highly publicised plans to try to enter the United States.

"The situation at the border has now reached a point of crisis. The lawlessness that continues at our southern border is fundamentally incompatible with the safety, security, and sovereignty of the American people," Trump said in a presidential memorandum.

"My Administration has no choice but to act."

Homeland Security Secretary Kirstjen Nielsen earlier said Trump had directed the Pentagon and the Department of Homeland Security to work with border state governors to determine how to deploy National Guard forces to assist Border Patrol agents.

The sudden action, which comes as lawmakers are out on Easter break, follows Trump taking to Twitter to rail against "ridiculous liberal" border laws, and warn of an inbound "caravan" of immigrants, threatening to axe the North American Free Trade Agreement if Mexico did not stop them.

Caravan leaders on Wednesday said most of the group -- about 80 per cent -- would now remain in Mexico, where authorities are working with individual migrants and families to get temporary papers.

"All they want is a place to live in peace, where they can work without having guns pointed at them, without being forced to join a gang," said Irineo Mujica, the head of migrant advocacy group People Without Borders (Pueblo sin Fronteras).

A handful of migrants with strong asylum claims will continue to the US border on their own, he said.

"Donald Trump wanted the world to crush us, to erase our existence. But Mexico responded admirably and we thank the government for the way it handled this caravan," Mujica said in the town of Matias Romero, in the southern Mexican state of Oaxaca.

Trump has ratcheted up pressure on both Congress and America’s southern neighbor Mexico in recent days to take action to stem illegal immigration.

"Until we can have a wall and proper security, we’re going to be guarding our border with the military," Trump said on Tuesday, referring to his financially challenged pet project to build a wall along the frontier.

The commander-in-chief’s seemingly off-the-cuff military directive caught Pentagon officials by surprise, and planners on Wednesday scrambled to find ways to support the edict.

Nielsen said the US continues to see "unacceptable levels" of illegal drugs, dangerous gang activity, transnational criminal organizations and illegal immigration flow across the southern border.

She and other administration officials boasted of a "Trump effect" that saw illicit border activity drop when Trump took office, but said the numbers of illegal border crossings had now risen back to previous levels. — AFP