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 Central American migrants traveling north on foot through Mexico, U.S. officials said on Thursday.

The plan calls for 800 to 1,000 more troops, most of them active-duty forces from the Army, to join a growing border mission called for by President Donald Trump, two officials said, speaking on the condition of anonymity because an official announcement had not been made.

Defense Secretary Jim Mattis could authorize the additional deployment as early as today, the officials said, adding that some troops could deploy within days, though others likely will arrive later.

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!”

Later, the president tweeted: “To those in the Caravan, turnaround, we are not letting people into the United States illegally. Go back to your Country and if you want, apply for citizenship like millions of others are doing!”

Critics said that a military solution would be costly and ineffective, and they accused Trump of trying to fan public fears over inflated security threats of the caravan to stoke his conservative base ahead of the Nov. 6 midterm elections. The caravan still is weeks from reaching the U.S. border, and Mexican authorities said the number of migrants has dwindled rapidly, from an estimate of 7,200 by the United Nations early in the week to 3,630 on Wednesday. The Mexican government said it had processed 1,700 asylum claims.

It is not clear what impact the troops would have, given that many of the migrants, if they reach the United States, would probably attempt to surrender to Border Patrol agents and seek legal asylum protections. U.S. officials said the troops would not conduct direct law enforcement but would instead play a supporting role. Those duties still were being defined, but are likely to include engineers who can oversee construction, aviation support and possibly doctors or lawyers who can assist migrants.

“It’s sad and irresponsible that the president would deploy the world’s strongest military against a group of unarmed migrants, including women and children,” said Kevin Appleby, a policy director at the Center for Migration Studies. “It demonstrates that the administration’s deterrence policies have failed and they are at the point of desperation in their immigration policies. They need a new approach, one that addresses regional challenges and the push factors from the (Central American) region. It’s a waste of taxpayer money.”

The new deployments, first reported by CNN, would constitute a separate and distinct mission from Operation Guardian Support, under which about 2,100 National Guard troops have been involved in border operations since the spring.

About 1,600 of those service members are in “border sectors,” with others in headquarters units, officials said this week.

The addition of active-duty forces could raise concerns among human rights groups, given that the caravan, which originated in Honduras, is made up largely of families, including children. U.S. officials said on Thursday morning that the additional forces are not expected to include any “trigger pullers.”

One U.S. official said the troops’ role will be designed so that the Posse Comitatus Act, a federal law that limits the use of the military for domestic law enforcement operations, is not violated. The official compared the deployment to active-duty troops assisting with hurricane relief.

The National Guard personnel already at the border are under direct 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.

RJ Hauman, government relations director at the Federation for American Immigration Reform, which advocates for lower immigration levels, said Trump deserves credit for the administration’s efforts to crack down on unauthorized immigration. But he added that a military solution to the Central American caravan would be “ineffective” and said the administration should focus on trying to get Congress to tighten asylum laws.

In most cases, children and families who apply for asylum are not legally allowed to be detained for more than a few weeks and are released into the United States as they await hearings before immigration judges, a process that could take more than a year due to lengthy backlogs.

Trump administration officials said there has been a rapid increase in asylum cases, especially from Central America, in recent years, and they have accused human smugglers of coaching immigrants to make false claims of persecution. Immigrant rights groups said the migrants are fleeing gang violence, organized crime and poverty.

“Sending the military down to the border will be ineffective in preventing these organized incursions so long as our asylum and immigration laws can be so easily abused,” Hauman said.

Both President George W. Bush and President Barack Obama dispatched National Guard troops to assist in border operations. The missions — including one from June 2006 to July 2008 and another from June 2010 to September 2011 — cost a total of $1.35 billion, according to a Government Accountability office report.

The report found that one of the primary benefits of the National Guard was to bridge gaps while new Border Patrol agents were hired and trained. But the report also found that Pentagon officials believed there was no comprehensive strategy at the border and worried about “mission creep” from Guardsmen. Department of Homeland Security officials, meanwhile, expressed concern that the Pentagon assistance was “ad hoc.”

Use of active-duty service has been especially sensitive since 1997, when a Marine on a drug surveillance mission shot and killed a high school student in the border village of Redford, Texas. The shooter and other Marines present were investigated, but no charges were brought against them. The U.S. government ultimately agreed to pay a $1.9 million settlement to his family.

This year, the Pentagon has examined 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.