The troops, which are expected to initially be stationed at ports of entry, will be deployed to provide additional security as the so-called "caravan of migrants" pushes toward the US-Mexico border.
The Customs and Border Patrol is pushing a surge in personnel in response to the caravan of Central American immigrants, which was still hundreds of miles from the USA border, three administration officials said.
In an interview Monday, Trump said the USA would build "tent cities" for thousands of Central American migrants making their way through Mexico to the United States.
Trump railed against illegal immigration to win the 2016 presidential election and has seized on the caravan of Central American migrants at campaign rallies in the run-up to next week's vote.
The massive deployment marks a sharp increase from initial estimates last week, when USA officials said about 800 active-duty troops would head south.
The Pentagon could send thousands more troops to the US-Mexico border, a US official said Monday, as President Donald Trump warned a caravan of Central American migrants that the military was waiting.
That's in addition to the more than 2,000 members of the National Guard already helping at the border now.
He added: "This is an invasion of our Country and our Military is waiting for you!"
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 first group passed through the spot via the river - wading or on rafts - and was advancing through southern Mexico.
In an interview Monday, Trump said the US would build "tent cities" for asylum seekers.
"By the end of the week, we will deploy over 5,200 soldiers to the southwest border", Air Force General Terrence O'Shaughnessy, head of the USA military's Northern Command, told reporters.
"We go for about a month, I stopped counting the days, we are helped by Almighty God and the good people you meet on the way", says this mother of a family.
Migrants are entitled under both US and global law to apply for asylum.
"Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border", Trump said.
The deployments thrust the military further into a political fight in which the president increasingly has sought to cast the migrants as a national security threat in the days leading up to the November 6 midterm elections.
The Pentagon already has sent 22 miles of concertina wire to the 1,954-mile-long border, he said, and has enough additional wire to cover 150 miles.
Trump said the United States would build "tent cities" to house migrants seeking asylum, rather than releasing them while they await court decisions. Those deployments, which primarily include military police and engineers, began Monday and will continue throughout the week, according to the document.