Donald Trump on Monday said he had alerted the military and federal border authorities that a US-bound migrant caravan from Central America was a national emergency, and that the United States would begin curtailing aid to Honduras, Guatemala and El Salvador.

Trump, in a series of posts on Twitter, gave no other details about his administration's actions. Representatives for the White House, the U.S. Border Patrol and the Pentagon did not immediately reply to a request for comment.

"Sadly, it looks like Mexico’s Police and Military are unable to stop the Caravan heading to the Southern Border of the United States," Trump wrote in a tweet, adding: "I have alerted Border Patrol and Military that this is a National Emergency."

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

Thousands of mostly Honduran migrants crowded into the Mexican border city of Tapachula over the weekend after trekking on foot from the Guatemalan border, defying threats by Trump that he will close the U.S.-Mexico border if they advanced, as well as warnings from the Mexican government.