"We'll have to see, we'll have to see what the future holds...the first thing that we brief is what is the threat against the border - people who try to violate the border", said the defense secretary to a reporter.
Some 400 migrants who broke away from the main caravan in Mexico City arrived in the border city of Tijuana on Tuesday by bus, according to a Reuters witness.
Because the route poses a host of dangers, such as attack by criminal gangs, numerous migrants say they feel safer travelling in numbers.
US Customs and Border Protection (CBP) said in a statement that it would close lanes at the San Ysidro and Otay Mesa crossings from Tijuana into California to allow the Department of Defence to install barbed wire and position barricades and fencing.
"CBP has been and will continue to prepare for the potential arrival of thousands of people migrating in a caravan heading towards the border of the United States", Pete Flores, director of San Diego field operations for Customs and Border Protection, said Monday in a statement.
Approximately 100 of an estimated 11,000 migrants, less than 1 percent of those US officials are tracking across four groups, may reach the USA border south of San Diego on Sunday. "They can put a million soldiers or whatever they want on the border, we are going to make the attempt to get across".
"We are well aware of everything Trump has been saying", said Maribel. But no other buses showed up and few trucks passed to pick them up, leaving many to walk the 60 miles (90 kilometers) to the state line.
Most plan to use their status as members of a persecuted class to request asylum in the United States as early as Thursday, the news outlet reported.
About 1,300 migrants in a second caravan were resting at a stadium in Mexico City, where the first group had stayed last week.
President Donald Trump, who has taken a firm stance against the caravan he calls an "invasion", has order the deployment of up to 9,000 troops to the border.