Mexicans from religious organizations hand out small bags containing water, toilet paper, diapers and medicine to Central American migrants who got a free ride from a motorist, in Xochiltepec, Mexico, Monday, Oct. 22, 2018. Motorists in pickups and other vehicles have been offering the Central American migrants rides, often in overloaded truck beds, as the group of about 7,000 people heads to the U.S. border. (AP Photo/Moises Castillo)

WASHINGTON — The Trump administration is expected to deploy additional U.S. troops to assist in security operations at the southern border in response to a caravan of migrants traveling north on foot through Mexico, three U.S. officials confirmed Thursday.

The plan calls for about 800 more troops, including some active-duty forces primarily from the Army, to join a growing border mission called for by President Donald Trump, one senior official said, speaking on the condition of anonymity because an official announcement had not been made.

The new deployments, first reported by CNN, would add to the estimated 2,100 National Guard troops already involved in border operations. About 1,600 of those service members are in “border sectors,” with others in a headquarters units, officials said this week.

In a Thursday morning tweet, Trump called again for changes to U.S. immigration laws, which he said “make it tough for us to stop people at the Border.” He added that he is “bringing out the military for this National Emergency. They will be stopped!”

Defense Secretary Jim Mattis is expected to authorize the additional deployment Thursday, officials said.

Numbers from the Department of Homeland Security, show a long-term trend towards fewer apprehensions near the Southwest border.

The addition of active-duty forces could raise concerns among human rights groups, given that the migrants in the caravan, which originated in Honduras, are made up largely of families, including children. U.S. officials said Thursday morning that the additional forces are not expected to include any “trigger pullers” and would instead include engineers to build new traffic barriers, aviation support, doctors and lawyers to provide legal representation.

But critics have said that Trump’s response to the caravan, estimated to be in the thousands, has been aimed at fanning public fears over security threats to rally his conservative base ahead of the Nov. 6 midterm elections.

Trump also has said in recent days that he is considering cutting aid to Honduras, Guatemala and, potentially, Mexico for failing to halt the caravan.

Yet it is not clear when the migrants would reach the U.S. border. Traveling by foot, they are probably weeks away, and the groups remains fluid and disorganized. Mexican authorities on Wednesday estimated their numbers at 3,630 – less than half of the 7,200 that the United Nations said were in the caravan earlier in the week. The Mexican government said it had processed 1,700 asylum claims.

The guardsmen already at the border are under orders from their respective state governors and remain under those governors’ control. Mattis issued a memo this year that prohibited them from interacting directly with “migrants or other persons detained,” and that directive is still in place, said Lt. Col. Dave Eastburn, a Pentagon spokesman.

The Defense Department has examined this year providing space to other federal agencies to run camps for migrants on specific military bases, and it said in a June memo to Congress that it would prepare to house up to 20,000 unaccompanied migrant children on bases in coming months.

That plan would have similarities to 2014, when the Obama administration housed about 7,000 unaccompanied children on three military bases. But to date, the U.S. government has not moved forward with opening any camps on military bases.

Officials at the Department of Homeland Security said Thursday that they did not know about the Pentagon’s plans to add more troops. White House officials did not respond to requests for comment.

The Washington Post’s Nick Miroff contributed to this report.