The Trump administration is preparing to send thousands of additional U.S. troops to the border with Mexico, U.S. officials said Monday, as President Trump likened a caravan of Central American migrants to "an invasion."

One Department of Homeland Security official with knowledge of the planning said 5,000 active-duty soldiers would be temporarily sent to the border, but two other U.S. officials cautioned that the final number had yet to be determined by the Pentagon. One of them said that the deployment will consist of "thousands" of U.S. troops.

It was not immediately clear why the scale of the mobilization increased fivefold from the 800 to 1,000 troops that defense officials were discussing last week. The additional personnel would join roughly 2,100 National Guard troops assigned to the border mission since April, and the combined force would be the largest deployment there in at least a decade.

Trump on Monday tweeted accusations about the caravan without citing any evidence.

"Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border," Trump said. "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!"

The White House has sought to make immigration the top issue of the Nov. 6 midterm elections, confident that Trump's hard-line enforcement message will continue to drive his conservative base to the polls and even draw some crossover appeal among more-moderate voters. The president has latched on to the migrant caravan, helping draw attention to the group and labeling it a national security threat.

White House press secretary Sarah Sanders said Monday that the administration is considering several administrative actions on the southern border, though she declined to describe the options publicly. Trump will do what "he deems necessary" on immigration, Sanders said.

Pentagon officials and Homeland Security officials are preparing a joint news conference Monday afternoon to describe the deployment in greater detail. A DHS official involved in the preparations said that plans have yet to be finalized but that the troop levels that were in consideration last week were not realistic.

"We've asked for capabilities, and DHS is looking to fill capabilities, and the [Pentagon] is in the process of determining which units to send and how many personnel it'll take, and that has not yet been determined," said the official, speaking on the condition of anonymity to discuss deliberations.

Navy Capt. Bill Speaks, a Pentagon spokesman, said Monday that it was premature to discuss the deployment because planning was still underway.

But photographs and videos released on the Pentagon's photo-sharing website showed that some new deployments to the border already were underway in a mission named Operation Faithful Patriot. One video depicted airmen from the Air Force's 3rd Airlift Squadron arriving at Fort Knox, Ky., to transport members of the Army's 89th Military Police Brigade to the border.

U.S. officials say the border deployment under consideration would not include "trigger pullers" tasked with arresting migrants or other enforcement duties. Rather, the troops would offer "logistical support" to the U.S. Border Patrol and other Homeland Security agencies, and would include construction brigades, aerial transport crews and medical staff.

The larger deployment was first reported by the Wall Street Journal.

Immigrant rights groups have accused Trump of demagoguery on the issue by inflating the size and security threat posed by the migrants, made up largely of families, including children.

The White House has put significant pressure on the government of Mexico to block the caravan's advance. The group has diminished from a peak of nearly 7,000 migrants, as some footsore travelers and parents with children have dropped out or fallen behind. At least 1,000 caravan members have applied for asylum in Mexico, authorities say.

Mexican President Enrique Peña Nieto on Friday offered temporary work permits, medical care and other benefits to migrants if they agree to register with authorities and remain in the Mexican states of Chiapas and Oaxaca, far from the U.S. border. But the core group of mostly Honduran migrants, which authorities estimate at 3,000 to 4,000, has rejected his entreaty and continued heading north toward the U.S. border.

The caravan remains at least 900 miles from U.S. territory, so its arrival is not imminent.

In an attempt to limit the caravan's size, Mexican police clashed Sunday with a smaller, separate group of Central Americans attempting to enter from Guatemala and catch up to the main group. At least one man was killed as police fired rubber bullets and tear gas.

Defense Secretary Jim Mattis said Sunday that he was finalizing details of the rules under which American troops deploying to the border would operate. Mattis said he would make certain that whatever materiel was needed would get to the border, noting that what the military will be providing includes construction items such as Jersey barriers. He said that his staff had been meeting in recent days to determine how many personnel would go but that the deployment would be "phased."

"On the border, we are preparing what we call defense support for civilian authorities," Mattis said.

The Washington Post's David Nakamura, Seung Min Kim, Alex Horton and Paul Sonne contributed to this report.