The first 100 active-duty troops have arrived at the U.S.-Mexican border to provide support for Border Patrol agents as a caravan of migrants slowly makes its way northward from Central America, a Defense Department official told Fox News on Thursday evening.

The troops are doing initial assessments at the port of entry in McAllen, Texas. The official said there are about 2,600 troops now at staging bases, largely in Texas, with several thousand more expected to flow in through the weekend and move into California and Arizona.

The Pentagon said more than 7,000 active-duty troops are being sent to the southern border, with more possible.

President Trump has said the number could reach 15,000.

Trump has drawn a hard line on immigration just ahead of the midterm elections.

Last week, officials indicated that 800 to 1,000 troops might be sent. On Monday, they announced that about 5,200 were being deployed. The next day, an Air Force general rejected a news report putting the figure at up to 14,000.

The troops going to the border areas of Texas, Arizona and California are a small fraction of the nation's roughly 1.3 million active-duty service members, and the mission is set to last only 45 days.

Gen. Terrence O’Shaughnessy, who as head of U.S. Northern Command is leading the military operation, dubbed “Operation Faithful Patriot,” has argued that the caravan is a potential threat, although he has not fully defined the nature of that threat.

Migrants from Central America, including many families and hundreds of children, are slowly moving north, though they remain hundreds of miles from the border. One group got into a violent confrontation with Mexican police at the border with Guatemala, throwing rocks.

“I think what we have seen is we’ve seen clearly an organization at a higher level than we’ve seen before,” O’Shaughnessy said. “We’ve seen violence coming out of the caravan and we’ve seen as they’ve passed other international borders, we’ve seen them behave in a nature that has not been what we’ve seen in the past.”

Trump said he told the military mobilizing at the border that if migrants try to throw rocks at soldiers, they should act as though the rocks are “rifles.”

Trump made the comments Thursday in a speech on immigration. He promised an executive order sometime next week that would ban migrants from claiming asylum if they cross the border illegally, and would set up vast tent cities that would hold anyone coming over the border.

U.S. immigration laws say migrants seeking asylum can do so no matter how they arrive at the U.S.

Fox News’ Jennifer Griffin and The Associated Press contributed to this report.