The US military reportedly turned down a White House request to build migrant detention centres as thousands walk towards to the Mexican border.
The Pentagon's apparent rejection, disclosed to Reuters by unnamed officials, illustrates the tension in the Trump administration over using military resources to fortify the border against illegal immigration.
In a rally ahead of Tuesday's midterm elections, President Donald Trump told supporters to "look at what is marching up - that's an invasion".
Last week, the US military announced more than 7,000 troops would go to the border with Mexico as thousands of central American migrants slowly march together towards the US.
Mexico's interior ministry estimated at the weekend that there were more than 5,000 migrants, from countries including Honduras, El Salvador and Guatemala, currently moving through southern Mexico either with the caravan or in smaller groups.
The soldiers are being deployed as part of a mission dubbed Operation Faithful Patriot to "harden" the southern border, supporting the border control and about 2,000 National Guard forces already sent there.
At a rally in Cleveland, Ohio, the president told supporters: "Democrats are inviting caravan after caravan of illegal aliens to flood into our country and overwhelm communities.
"They try and play it down like it's not that many people. You're looking at a bridge with thousands and thousands. We're building the wall, we've already started."
He added: "You think we're letting that caravan in. You can forget it."
Mr Trump said last Thursday that US troops should treat rocks thrown by migrants like firearms attacks.
"Consider it a rifle," he said. "When they throw rocks like they did at the Mexico military police, consider it a rifle."
But a day later he appeared to row back, telling reporters if agents or soldiers "are going to be hit in the face with rocks, we're going to arrest those people. That doesn't mean shoot them".
Immigration is a big election issue in the midterms, and Mr Trump claimed Democrats encouraged chaos at US borders because it was good politics.
"Democrats want to invite caravan after caravan of illegal aliens into our country," he said.
A large group of migrants has vowed to push on towards Mexico City as troops began erecting barbed wire fencing .
They have been walking for three weeks and some days demand treks of more than 100 miles (160km).
They reached a gymnasium in Cordoba where they held a vote to cover the 178 miles (285km) to Mexico City on Monday by walking and hitching rides.
Many of the people still with the caravan have now covered more than 800 miles (1,200km) since setting off from Honduras on 13 October.
Calls have grown in recent days for buses to transport scores of people to the Mexican capital for medical treatment, but no buses have arrived.
Some migrants have broken away from the caravan and gone ahead but many feel their chances of getting into the US are better if they go in numbers.
Many migrants are fleeing gang violence and financial hardship in their home countries.
Meanwhile, NBC and Fox News say they will stop airing President Trump's campaign advert that featured an immigrant convicted of murder. CNN had already rejected the ad, declaring it racist.
The ad linked Luis Bracamontes, convicted of killing two sheriff's deputies in California in 2014, to the migrant caravan.
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