WASHINGTON (Reuters) - Thousands of U.S. troops may be headed to the border with Mexico ahead of a caravan of migrants trekking toward the United States, U.S. officials told Reuters on Monday, offering much higher estimates than the 800 to 1,000 initially forecast.
The Pentagon declined comment on potential troop numbers, saying planning was still underway for a mission that risks drawing the U.S. military into a politically charged operation just days ahead of Nov. 6 mid-term congressional elections.
President Donald Trump, who has seized upon the Central American migrant caravan in campaign rallies ahead of the vote, said on Twitter that the military would be waiting for the procession -- suggesting a far more direct role in confronting the migrants than U.S. defense officials have previously suggested.
"Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border," Trump tweeted.
"Please go back, you will not be admitted into the United States unless you go through the legal process. This is an invasion of our Country and our Military is waiting for you!" he added.
U.S. officials told Reuters last week the military will have not have an active law enforcement role, instead sending engineers, pilots and other support staff including some that may be able deal with crowd control on the U.S. side of the border. Pentagon officials declined comment on Monday.
Last week, Defense Secretary Jim Mattis authorized the use of troops and other military resources at the U.S-Mexico border. U.S. officials told Reuters they would begin deploying as soon as Tuesday and their mission was authorized through mid-December.
That authorization could be extended, they noted.
Mattis' authorization last week allowed the military to provide "mission-enhancing capabilities" to U.S. Customs and Border Protection to help build temporary barriers and housing.
On Sunday, Mattis told reporters that some construction materials were already being moved to the border, including barriers. He said the Department of Defense was still planning the logistics and details of the deployment.
Mattis suggested the mission would take time to develop, saying it would be a "phased" operation. U.S. officials have told Reuters the support mission would be coordinated by the U.S. military's Northern Command.
Trump, who campaigned against illegal immigration to win the 2016 U.S. presidential vote, has seized on this caravan in the run-up to the congressional elections, firing up support for his Republican Party, which is seeking to maintain control of Congress.
If the Republicans lose Congress, it could make it much harder for Trump to pursue his policy agenda in the remaining two years of his term in office.
Estimates on the size of the caravan vary from around 3,500 to more than double that. Some migrants have abandoned the journey, deterred by the hardships or the possibility of making a new life in Mexico. Others joined it in southern Mexico.
Trump's decision to call in the military appears to be a departure from past practice, at least in recent years, in which such operations were carried out by National Guard forces -- largely part-time military members who are often called upon to serve in response to domestic emergencies.
There are 2,100 U.S. National Guard forces at the border already, sent after a previous Trump request in April.
The decision to send active duty forces this time gives the Pentagon the ability to more rapidly mobile greater capability than would be immediately available with the Guard, officials tell Reuters.
(Reporting by Phil Stewart; Editing by Chizu Nomiyama and Alistair Bell)