"And if the people who are fleeing persecution and violence enter Mexico, they need to be provided access to the Mexican asylum system, and those entering the United States need to be provided access to the American asylum system".
Another, smaller caravan earlier this year dwindled greatly as it passed through Mexico, with only about 200 making it to the California border.
General Terrence O'Shaughnessy, the head of US Northern Command, said 800 US troops were already en route to the Texas border and 5,200 would be headed to the south-west region by the week's end, far higher than the 800 to 1,000 initially forecast.
When asked if the USA military was being used as part of a political stunt, O'Shaughnessy noted that the US military routinely partners with the Department of Homeland Security on a host of operations. Those migrants clashed with Mexican authorities determined not to let the caravan grow or be repeated.The new group, whose members called themselves a second caravan, gathered on the worldwide bridge leading from Tecun Uman to Mexico. He said the caravans was "different" than those seen by the United States in the past, adding that they were better organized.
That caravan is estimated to contain 4,000 people - but the number is slowly shrinking as tired travelers drop out or apply for protected status in Mexico, spurring the caravan's leaders to make the request for help with transportation to Mexico's capital, according to the Associated Press.
The move represents a massive military build-up along the border, where some 2,000 National Guardsmen are already working to provide help to overwhelmed authorities.
McAleenan said the aim of the operation was to deter migrants from crossing illegally, but he conceded his officers were overwhelmed by a surge of asylum seekers at border crossings.
Victor Argueta, 54, of Santa Barbara, Honduras, said he and his wife had spent two nights sleeping on the worldwide bridge between Tecun Uman, Guatemala, and Ciudad Hidalgo, Mexico, before eventually crossing the river on a raft.
At least 1,895 have applied for refugee status in Mexico, and hundreds of others have accepted assisted returns to their country of origin.
The military operation drew quick criticism.
Military personnel are legally prohibited from engaging in immigration enforcement. "We will continue to adjust the number and inform you of those", O'Shaughnessy told reporters.
Sen. Ed Markey, D-Mass., who has been a vocal opponent of Trump's immigration policies, voiced his objection Monday to deploying more than 5,000 troops.
The announcement comes just days after President Trump declared the migrant caravan making its way to the USA a "National Emergency". "This is an invasion of our Country and our Military is waiting for you!"
The troops are expected to perform a wide variety of functions such as transporting supplies for the Border Patrol, but not engage directly with migrants seeking to cross the border, officials said.
Trump said anyone wanting to get into the United States must do so through the legal process. US officials have told Reuters the support mission would be coordinated by the USA military's Northern Command. "But please know that is in addition to the 2,092 that are already employed from our national guard troops".