said Sunday that temporary identity numbers had been issued to more than 300 migrants, which would allow them to stay and work in Mexico.
"The president has made it clear that border security is national security", O'Shaughnessy said, as he detailed a much larger deployment that the 800 to 1,000 troops predicted by USA officials last week.
U.S. President Donald Trump has hardened his stance on immigration ahead of November 6 congressional elections.
But critics say Trump is politicizing the military, deploying them as a stunt to drive Republican voters to the polls without any real national security threat. More than 1,500 of the original group have applied for asylum in Mexico.
The Pentagon's "Operation Faithful Patriot" was described by the commander of U.S. Northern Command as an effort to help Customs and Border Protection "harden the southern border" by stiffening defenses at and near legal entry points.
Trump also said the group includes "Gang Members and some very bad people".
Customs and Border Protection is pushing a surge in personnel in response to the caravan of Central American immigrants, which is still hundreds of miles from the United States border.
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. We're going to hold them, we're going to build tent cities, we're gonna build tents all over the place.
Trump denied his focus on the caravan is meant to help Republicans in next week's midterms, saying, "This has nothing to do with elections".
Several thousand Central American migrants planned to resume their trek through southern Mexico before dawn Monday, while authorities in that country and Guatemala tried to sort out the killing of a migrant at a border crossing.On Sunday, while the band of migrants was resting and reorganizing in Tepanatepec, several hundred in another group more broke through border barriers in the Guatemala town of Tecun Uman just as members of the caravan did more than a week earlier.
Trump says that the US was "going to be prepared" and that the migrants are "not coming into our country". The Pentagon says an additional 5,200 active-duty troops could join them.
Trump told reporters at the White House on Wednesday that "we'll go up to anywhere between 10 and 15,000 military personnel on top of Border Patrol, ICE and everybody else at the border". "If you want to wait, they don't usually get asylum". U.S. Transportation Command posted a video on its Facebook page Monday of a C-17 transport plane that it said was delivering Army equipment to the Southwest border in support of the operation.
The US President said that those illegal immigrants who enter the United States and request for asylum will not be released as long as their cases will not be considered by the immigration court.
The projected US deployment is already roughly the same size as the USA military contingent in Iraq.