President Donald Trump said the U.S. will close its southern border with Mexico if needed, a day after U.S. agents shot several rounds of tear gas at migrants after some tried to breach a border fence.

“Mexico should move the flag waving Migrants, many of whom are stone cold criminals, back to their countries,” Trump said Monday on Twitter. “Do it by plane, do it by bus, do it anyway you want, but they are NOT coming into the U.S.A. We will close the Border permanently if need be.”

The Mexican peso fell as much as 0.3 percent after Trump’s tweet, weakening to as low as 20.4624 per dollar, one of only a handful of decliners in emerging markets Monday.

U.S. border officials on Sunday temporarily closed the San Ysidro port of entry between Tijuana and San Diego, one of the busiest border crossings in the world, “to ensure public safety” after agents shot several rounds of tear gas at migrants, leaving children screaming and coughing, the Associated Press reported.

An estimated 8,200 migrants from the so-called caravans heading to the U.S. from Central America are now in Mexico, authorities say. Trump has vowed to shut down the southern border with lethal force if necessary, and is calling on Mexico to intervene.

Immigration — an issue Trump has relied on to rev up his base — has proved legislatively thorny. Democrats and even some lawmakers in Trump’s own party have resisted his calls to fund his proposed border wall.

Trump wants migrants to remain in Mexico while their U.S. asylum cases are adjudicated. Olga Sánchez Cordero, Mexico’s incoming interior minister, as well as a spokesman for incoming President Andres Manuel Lopez Obrador, have said Mexico won’t accept that proposal.

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