WASHINGTON (AP) — Active duty military troops will begin arriving in El Paso, Texas, and San Diego on Thursday evening, in what defense officials said is the first batch of the new forces being deployed to secure the southern border.

The Pentagon announced on Wednesday that about 1,500 troops were being sent to the border this week, as the department scrambles to put in motion President Donald Trump’s executive order demanding an immediate crackdown on immigration.

U.S. officials said they expect additional troops to be ordered to deploy in the next few days as defense and homeland security leaders iron out requests for more support.

The officials said it’s not yet clear how many more service members would get tapped in the near future, but they would include active duty, National Guard and Reserves, and come from land, air and sea forces. Other defense and military officials this week estimated that the additional number deployed could be in the thousands.

The troops announced Wednesday include about 1,000 Army soldiers from a variety of units and 500 Marines from Camp Pendleton in California.

Officials said Thursday that they expect the bulk of them to be in El Paso — including Fort Bliss — or in San Diego by Friday, where they will get their mission assignments and prepare to spread out along the border. The officials spoke on condition of anonymity to provide details on troop movements.

There were already about 2,500 Guard and Reserve forces deployed to the border, and the new 1,500 would add to that total. But officials noted that given the length of the nearly 2,000-mile border with Mexico, it will take additional forces to help put large rolls of concertina wire barriers in place and provide needed transportation, intelligence and other support to the Border Patrol.

As of Thursday there were still no requests for the use of military bases to house migrants or for troops to be used for law enforcement duties.