By Phil Stewart and Makini Brice

WASHINGTON (Reuters) – The U.S. military has received a request from the Department of Homeland Security for active-duty troops on the U.S.-Mexico border, a U.S. official said on Thursday, after President Donald Trump said he was “bringing out the military” to guard against a caravan of Central American migrants trekking through Mexico.

The U.S. official, speaking to Reuters on condition of anonymity, said the U.S. military was examining the request that could require deploying between 800 and 1,000 active-duty troops to the border to assist with logistics and infrastructure.

The U.S. official said that any troops deployed to the border would not be involved in “law enforcement” activities, something that would be prohibited by a federal law dating to the 1870s.

That law restricts the use of the Army and other main branches of the military for civilian law enforcement on U.S. soil unless specifically authorized by Congress. But the military can provide support services to law enforcement and has done so on occasion since the 1980s.

Some specific statutes authorize the president to deploy troops within the United States for riot control or relief efforts after natural disasters.

The Pentagon did not immediately respond to a request for comment.

Trump has taken a hard line toward immigration – legal and illegal – since becoming president last year. On Monday, Trump said he had alerted the Border Patrol and the U.S. military that the migrant caravan was a national emergency.

Despite raising Trump’s ire, thousands of Central American men, women and children seeking to escape violence, poverty and government corruption in their home countries continued their journey toward the distant U.S. border. Under a full moon early on Thursday, they walked from Mapastepec, close to the Guatemala border in southern Mexico. A town official said there had been 5,300 migrants in Mapastepec on Wednesday night.

A second group of more than a thousand people has started a similar journey from Guatemala.

“I am bringing out the military for this National Emergency. They will be stopped!” Trump wrote on Twitter, referring to the migrants.

White House officials did not immediately respond to requests for comment on Trump’s comments regarding a military deployment and a national emergency.

Trump and his fellow Republicans have sought to make the caravan and immigration major issues ahead of the Nov. 6 U.S. congressional elections in which the party is trying to maintain control of the House of Representatives and the Senate.

It is not new territory for Trump, who pledged during the 2016 presidential race to build a wall along the southern U.S. border with Mexico. However, funding for his signature campaign promise has been slow to materialize even though his party controls Congress and the White House.

In April, frustrated by lack of progress on the wall, Trump ordered the National Guard to help secure the border in four southwestern states. There are currently 2,100 National Guard troops along the borders of Texas, New Mexico, Arizona and California.

Also in April, Trump raised the prospect of sending active-duty military forces to the border to block illegal immigration, raising questions in Congress and among legal experts about troop deployments on American soil.

(Reporting by Makini Brice; Additional reporting by Delphine Schrank in Mapastepec, Mexico; Editing by Bernadette Baum and Will Dunham)