This comes as thousands of migrants march through Latin America toward the USA border despite the US threatening to cut-off aid from those involved.
Trump also suggested that the migrants hoping to enter the US are linked to crime and drugs that he said are "pouring in", and he called on Mexico to stop the "onslaught". "We have the right to protect the citizens of this country".
The Washington Post reported this week that Border Patrol agents arrested 16,658 family members in September, the highest one-month total on record and an 80 per cent increase from July, according to unpublished Department of Homeland Security statistics. "I'm stunned [by] what this country has become".
In a string of tweets, Mr Trump also said the issue was more important to him than the new trade deal with Mexico to replace the North American Free Trade Agreement pact.
President Trump warned Thursday he will use the US military to close the southern border with Mexico if officials there don't stop a growing migrant caravan that's swelled in size to about 4,000 people.
The latest caravan is now estimated to include approximately 4,000 men, women and children from Honduras and Guatemala.
Heeding Trump's warning, the Mexican government has sent 500 additional federal police to its border with Guatemala in anticipation of the migrant caravan's arrival.
The president's warning echoes an earlier tweet on Wednesday, in which he again blasted Democrats for refusing to approve tighter border control and immigration laws.
Meanwhile, a Honduran caravan of migrants headed for the United States border has grown to 4,000.
Trump has made migrant caravans a symbol of all that is wrong with USA immigration policies.
The migrants say they organized for the long and hard journey on social media, but Washington suspects the group has been organized as a deliberate provocation.
Frustrated by Congress' failure to fully fund his proposed wall at the border with Mexico, Mr Trump in April ordered National Guard personnel to help the Department of Homeland Security secure the border in four southwestern U.S. states.