WASHINGTON – Just one week shy of the midterm elections, the Pentagon will deploy at least 5,200 active-duty troops to the U.S.-Mexico border in an effort to prevent members of a migrant caravan from illegally entering the country, the Department of Defense announced Monday.

About 2,100 National Guard troops already were fanned out across the border under an order from President Donald Trump this year. In recent weeks, the president has been warning repeatedly about the dangers posed by the caravan of mostly Central American migrants, which stands at about 3,500 people after 1,700 of them filed asylum applications in Mexico or accepted assistance to return to their home countries.

Administration officials said last week that they were considering a plan to send up to 1,000 active-duty troops to the border, but that deployment, dubbed Operation Faithful Patriot, will now surpass 5,200, said Gen. Terrence O’Shaughnessy, commander of U.S. Northern Command. He said the number of troops could rise depending on the demands placed on U.S. Customs and Border Protection agents manning the border.

"That is just the start of this operation," O’Shaughnessy said during a news conference at CBP headquarters in Washington on Monday. "Border security is national security."

The troops will not conduct law enforcement activities, but some will be armed as they provide support to Border Patrol agents along the border.

National Guard units have already been assisting by monitoring video surveillance feeds to direct Border Patrol agents manning the vast stretches between U.S. ports of entry. The new deployment of active-duty troops will include helicopter and other aviation units armed with night vision technology to help identify anyone trying to illegally cross the border, and to deploy CBP agents to apprehend them, O’Shaughnessy said.

“We will be able to spot and identify groups, and rapidly deploy,” he said.

The operation will also include engineering units to build temporary barriers, lay out concertina wire at ports of entry, and construct temporary housing for U.S. personnel, he said. About 800 troops are already en route to Texas from Fort Campbell and Fort Knox, and more will follow this week headed toward Arizona and California.

Typically, migrant caravans travel in numbers to seek safety and avoid risks such as kidnap, rape and extortion. When the last migrant caravan reached the U.S. border in April, a majority of people presented themselves at ports of entry to request asylum, a legal way to enter the United States.

A Homeland Security official, who spoke on condition of anonymity because plans were not yet finalized, said the president is expected to deliver a speech this week to outline further actions to halt the migrant caravan, which could include limiting, or halting, the ability of migrants to request asylum. That move would be predicated on national security arguments similar to those used to enact Trump’s travel ban last year on certain individuals from Muslim-dominated countries and would meet the same legal challenges by immigration advocacy and civil rights organizations.

White House spokeswoman Sarah Sanders declined on Monday to detail other aspects of the new border security plan, and would not confirm whether Trump will deliver a speech on the topic this week.

"There are a lot of options that are being discussed right now," she said.

Trump on Monday again claimed that the migrant caravan may have "many gang members" and "some very bad people." Reporters from USA TODAY and other media outlets, including The Associated Press, have not seen the presence of such individuals. Supporters of the caravan have denied such allegations. Many of the migrants are families traveling with children.

"Please go back, you will not be admitted into the United States unless you go through the legal process," he tweeted. "This is an invasion of our Country and our Military is waiting for you."

Critics accused Trump of using the Pentagon as a tool in his political game by drumming up anti-immigrant fears to rally his political base in the lead-up to the midterm elections.

"At a moment we need a president that helps the nation heal and unite, we have one that is ripping us apart and using racism, xenophobia and antisemitism as strategic weapons in the run up to the elections," tweeted Frank Sharry, executive director of America's Voice, an immigration advocacy group.

U.S. Customs and Border Protection Commissioner Kevin McAleenan, who has 16,500 Border Patrol agents along the southwest border, said the additional manpower is needed because the region is in a state of crisis. He said an average of 1,900 people a day have been arriving illegally or without official documents for the last three weeks, straining his agency's resources to process illegal border crossers and process legal ones.

Monday's news conference did not address speeding up the process by which migrants can request asylum, which they are legally allowed to do. McAleenan said on Monday that Central American migrants were already given an opportunity to request asylum in Mexico, calling into question their motivations behind ignoring those requests and marching on toward the U.S.

"Our message to the organizers and the participants in the caravan is simple: As the president and (Department of Homeland Security Secretary Kirstjen) Nielsen have made clear, we will not allow a large group to enter the United States in an illegal and unsafe manner," he said.

The administration's moves are the latest to show a robust response to the migrant caravan as the midterm elections draw closer. Nielsen on Friday showed off a recently completed section of border fence in Calexico, California, that including a plaque proclaiming the "completion of the first section of President Trump’s border wall." That section had been deemed a priority and funded during the Obama administration.

On the same day, McAleenan toured ports of entry south of San Diego and explained how additional troops would help secure the border from any attempted rush by the migrants to rush the border. But McAleenan, who described the oncoming caravan as a "law enforcement situation," conceded that despite the added manpower, his agency has no way to speed up the process by which migrants can legally request asylum.

That's what happened when the last migrant caravan reached the U.S. border in April – 122 were caught trying to enter the country illegally, but 401 presented themselves at ports of entry to request asylum, according to data from U.S. Citizenship and Immigration Services.

Contributing: David Jackson and Tom Vanden Brook in Washington.