But across his administration there was no indication of any action in response to what he tweeted was a "National Emergy" (sic).

For hours on Monday, White House officials were unable to provide an explanation for the president's threats, which reflected both his apparent frustration with the migrant caravan and his determination to transform it into Republican election gains.

Federal agencies said they'd received no guidance on the president's declaration, issued as he attempts to make illegal immigration a focus of next month's midterm elections.

If Trump should follow through with his threat to end or greatly reduce US aid, that could worsen the poverty and violence that are a root cause of the migration he has been railing against, critics said.

Trump tweeted, "Sadly, it looks like Mexico's Police and Military are unable to stop the Caravan heading to the Southern Border of the United States." He added without evidence that "criminals and unknown Middle Easterners are mixed in."

"I have alerted Border Patrol and Military that this is a National Emergy," he wrote. "Must change laws!"

Associated Press journalists travelling with the caravan for more than a week have spoken with Hondurans, Guatemalans and Salvadorans but have not met any of the "Middle Easterners" that Trump claimed had "mixed in" with the Central American migrants. It was clear, though, that more migrants were continuing to join the caravan.

At a campaign rally in Houston on Monday night, he falsely accused Democrats of "encouraging millions of illegal aliens to break our laws, violate our borders and overwhelm our nation".

Trump for months has sought to use foreign aid as a cudgel more broadly, threatening to withhold humanitarian and other aid from "enemies of America" and using it to pressure foreign governments to bend to his will. On Monday, he said he would be making good on his threat.

"Guatemala, Honduras and El Salvador were not able to do the job of stopping people from leaving their country and coming illegally to the US We will now begin cutting off, or substantially reducing, the massive foreign aid routinely given to them," he wrote.

However, it was unclear whether the president's tweets had any policy implications.

Asked what the administration was doing to operationalise the president's tweet, White House spokeswoman Sarah Huckabee Sanders told reporters Monday evening that "we're continuing to look at all options on the table".

"The president wants to make sure we're doing everything we can to secure and protect our borders and that's exactly what he's been talking about," she said.

It is Congress, not the president, that appropriates aid money. The White House would have to notify Congress if it wanted to cut or reallocate aid, which could delay or complicate the process.

Australian Associated Press