Defense Secretary Jim Mattis is expected to sign an order as early as Thursday sending 800 or more troops to the southern border to support the Border Patrol, a U.S. official said.

Mattis is responding to a request from President Donald Trump, who says he's "bringing out the military" to address what he's calling a national emergency at the border. Thousands of Central American migrants are now making their way through Mexico in hopes of reaching the United States.

The U.S. official was not authorized to speak publicly because not all details of the military arrangement had been worked out, and so spoke on condition of anonymity.

The additional troops are to provide what one official described as logistical support to the Border Patrol. This would include a variety of things such as vehicles, tents and equipment, and perhaps medical support. It was not immediately clear how many, if any, of the extra troops would be armed. Because they would not be performing law enforcement duties they would not be in violation of the Posse Comitatus Act, which prohibits the federal government from using the armed forces in a domestic police role.

There already are about 2,000 National Guard troops assisting at the border under a previous Pentagon arrangement.

Trump announced that he's "bringing out the military" in a tweet Thursday morning, blaming Democrats for laws he said make it too hard to stop people at the border. He said those trying to enter the U.S. "will be stopped!"

Trump tweeted Monday that he'd alerted the military that the situation was a "National Emergy" — but the Pentagon said then that it had received no new orders to provide troops for border security.

Trump told a rally crowd in Wisconsin on Wednesday that the military was "all set."

The president's tweets reflect an effort to thrust immigration politics into the national conversation in the closing weeks of the congressional election campaign. He and his senior aides have long believed the issue — which was a centerpiece of his winning presidential campaign — is key to revving up his base and motivating GOP voters to turn out in November.