Trump, who has made curtailing immigration and building a border wall on the Mexican border a key platform, has previously threatened to shut off aid and dispatch troops there.
In a string of tweets on Thursday, Trump also appeared to link the issue to trade and a newly minted deal with Mexico to replace the North American Free Trade Agreement pact that is awaiting ratification.
"The assault on our country at our Southern Border, including the Criminal elements and DRUGS pouring in, is far more important to me, as President, than Trade or the USMCA. Hopefully Mexico will stop this onslaught at their Northern Border," Trump wrote, referring to the newest trade deal known as the US-Mexico-Canada Agreement.