(BBC) The Pentagon is sending up to 5,200 troops to the border with Mexico to cope with an expected migrant influx.
General Terrence O’Shaughnessy said Operation Faithful Patriot would focus on Texas, Arizona and California.
President Donald Trump earlier said the thousands of Central American migrants heading north would find the US military waiting for them.
There are already 2,100 National Guard members at the border, sent after a previous request by Mr Trump in April.
Before Gen O’Shaughnessy spoke at Monday’s news conference, US Customs and Border Protection commissioner Kevin McAleenan told reporters the migrants were estimated to be several weeks away from reaching the US border.
The figure of 5,200 military personnel, some of whom will be armed, is higher than the 800 troops it was initially predicted would be sent.
The total military deployment means the number of troops at the south-western border will exceed the US troops currently in Syria and Iraq, the Wall Street Journal notes.
Mr Trump tweeted on Monday: “Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border.
“This is an invasion of our Country and our Military is waiting for you!”
On Sunday, Defence Secretary James Mattis told reporters that construction materials, including barriers, were being moved to the border as part of the deployment.
He said the Pentagon was planning logistics and details.
“Please go back, you will not be admitted into the United States unless you go through the legal process.
Many of the migrants say they plan to seek asylum in the US.
There is a legal obligation under international law to hear asylum claims from migrants who have arrived in the US if they say they fear violence in their home countries.
Those seeking asylum must be fleeing due to a serious fear of persecution in their home country. Under international law, these are considered refugees.
If an asylum seeker enters the US illegally, they are still entitled to a hearing of their claim.
Economic migrants are those seeking a better quality of life – and even if they are fleeing devastating poverty, they are not considered refugees and do not have the same protections.
Mr Trump is not the first president to send troops to the border with Mexico.
President Barack Obama sent some 1,200 National Guard soldiers to guard the boundary, while President George W Bush deployed about 6,000 troops to help Border Patrol in what was called Operation Jump Start.
Both deployments lasted for about a year.