President Donald Trump sent another warning to the migrant caravan about 1,000 miles away from the United States, telling them again to try to immigrate legally and threatening that if they continue toward the border, "our Military is waiting for you!"
U.S. officials last week said about 800 active-duty troops would be sent to provide assistance - mainly in the form of logistical support - to border guards working along the frontier.
The announcement comes just days after President Trump declared the migrant caravan making its way to the US a "National Emergency".
President Donald Trump, eager to focus voters on immigration in the lead-up to the mid-term elections next week, escalated his threats against the caravan, tweeting: "This is an invasion of our Country and our Military is waiting for you!"
The first group passed through the spot via the river - wading or on rafts - and was advancing through southern Mexico.
General Terrence O'Shaughnessy, the head of U.S. Northern Command, defended the operation at a briefing on Tuesday.
"Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border", Trump wrote without providing evidence, doubling down on the hardline anti-immigrant rhetoric that helped fuel his 2016 election victory.
But any migrants who complete the long trek to the southern US border already face major hurdles - both physical and bureaucratic - to being allowed into the United States.
The caravan of 3,500 has shrunk from a peak of about 7,200 migrants a week ago but a second caravan of about 600 had formed and was clashing with federal police on a bridge from Guatemala to Mexico.
The troop deployment, one week before the U.S. midterm elections, appears to be the largest U.S. active-duty mobilization along the U.S. -Mexico boundary in decades and amounts to a significant militarization of American border security.
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The deployments thrust the military further into a political fight in which the president increasingly has sought to cast the migrants as a national security threat in the days leading up to the November 6 midterm elections.
The troops will join the more than 2,000 National Guardsmen that Trump has already deployed to the border. But there already is a bottleneck of would-be asylum seekers waiting at some USA border crossings to make their claims, some waiting as long as five weeks.
The Pentagon last week approved a request for additional troops at the southern border.
The White House has reportedly been considering a more extensive shutdown of the border, including an executive order that would ban asylum applications by those who reach the U.S. The troops will include military police, combat engineers and others helping on the border.
"If you wanna wait, they don't usually get asylum".
"These migrants need water, diapers and basic necessities - not an army division", Shaw Drake, policy counsel for the ACLU's Border Rights Center in El Paso, said in a statement.
The caravan remains at least 900 miles from USA territory, so its arrival is not imminent.
"Please go back", the president tweeted Monday.
The projected US deployment is already roughly the same size as the USA military contingent in Iraq.