WATCH: President Trump on why tear gas was used against children at US/Mexico border.

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As photos of migrant children fleeing barefoot from tear gas prompted online outcry, U.S. President Donald Trump defended the practice to reporters outside the White House Monday.

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MAP: Where the migrant caravan began, and why people are fleeing these countries

He told reporters “some very tough people” were trying to break in to the U.S., according to The Washington Times.

“Here’s the bottom line — nobody’s coming in to our country unless they come in legally,” he said.

Earlier in the day, he sent a similar message on Twitter, writing: “Mexico should move the flag waving Migrants, many of whom are stone cold criminals, back to their countries. Do it by plane, do it by bus, do it anyway you want, but they are NOT coming into the U.S.A.”

The president also threatened to permanently close the border with Mexico.

Trump’s statements come a day after a peaceful march in the Mexican city of Tijuana descended into chaos. U.S. agents fired tear gas into Mexico to stop some migrants as they attempted to breach the border through fencing and wire separating the two countries.

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Thirty-nine migrants were arrested.

WATCH: Trump says migrant caravan has ‘grabbers’ who take children as protection when rushing border

The gas reached hundreds of migrants, many of whom were photographed running away from the smoke.

They included young children running barefoot, some wearing only diapers.

A migrant girl from Honduras, part of a caravan of thousands traveling from Central America en route to the United States, cries after running away from tear gas thrown by the U.S. border control near the border wall between the U.S. and Mexico in Tijuana, Mexico, Nov. 25, 2018. Kim Kyung-Hoon/Reuters

A migrant family from Honduras, part of a caravan of thousands traveling from Central America en route to the United States, runs from tear gas released by U.S. border patrol near the fence between Mexico and the United States in Tijuana, Mexico, Nov. 25, 2018. Kim Kyung-Hoon/Reuters

Lurbin Sarmiento, 26, of Honduras, was with her four-year-old daughter and was shaken by what had unfolded.

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“We ran, but the smoke always reached us and my daughter was choking,” Sarmiento told The Associated Press.

WATCH: Trump speaks about migrant caravan

She said she never would have gotten that close with her daughter if she thought there would be gas.

U.S. soldiers and U.S. border patrols fire tear gas towards migrants, part of a caravan of thousands traveling from Central America en route to the United States, from the U.S.side of the border fence between Mexico and the United States in Tijuana, Mexico, Nov. 25, 2018. Kim Kyung-Hoon/Reuters

A migrant, part of a caravan of thousands from Central America trying to reach the United States, wears a homemade gasmask as he walks near the border fence between Mexico and the United States in Tijuana, Mexico, Nov. 25, 2018. Hannah McKay/Reuters

Another mom, Cindy Milla, told The Wall Street Journal that her child fainted.

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“I felt that my face was burning, and my baby fainted,” she said. “I ran for my life and that of my children.”

One woman collapsed after the tear gas was fired, while a Reuters reporter noted seeing two babies were visibly distraught.

WATCH: Mother in migrant caravan describes running from tear gas launched at U.S. border

The U.S. decision to fire tear gas into the crowd was criticized online, including by several politicians and advocacy organizations.

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“Tear gassing families seems unnecessary,” Lee Gelernt, the deputy director of the ACLU Immigrants’ Rights Project, told CNN.

WATCH: Tensions flare at U.S.-Mexico border

Gavin Newsom, Governor-elect of California, also slammed the move.

“These children are barefoot. In diapers. Choking on tear gas. Women and children who left their lives behind — seeking peace and asylum — were met with violence and fear,” he tweeted Sunday night.

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As Vox noted, tear gas is considered a “chemical weapon” under international law — but it isn’t illegal for domestic law enforcement. This case is complex, because tear gas canisters were thrown across the border.

U.S. Customs and Border Protection said Monday that the use of tear gas will be reviewed.

READ MORE: Mexico to increase security at U.S. border, says it will deport migrants who try to cross

Homeland Security secretary Kirstjen Nielsen said in a statement that U.S. authorities will continue to have a “robust” presence along the southwest border and that they will prosecute anyone who damages federal property or violates U.S. sovereignty.

“DHS will not tolerate this type of lawlessness and will not hesitate to shut down ports of entry for security and public safety reasons,” she said.

— With files from The Associated Press, Reuters

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