The commander in chief said Wednesday that as many as 15,000 troops could be headed to the U.S.-Mexico border, which would top the number military leaders have said they're planning to send.

President Donald Trump told reporters at the White House that "we'll go up to anywhere between 10,000 and 15,000 military personnel" on the border as a group of migrants makes its way north on foot through Mexico. The military personnel would be on top of the border patrol and immigration and customs enforcement agents from other departments already staged there, he added.

Air Force Gen. Terrence O'Shaughnessy, head of U.S. Northern Command, said Tuesday that the number of troops participating in Operation Faithful Patriot could rise above the 5,239 who have deployed or are preparing to deploy to the border. But he disputed reports that the number could go as high as 14,000.

"I honestly don't even know where that came from," O'Shaughnessy said. "That is not in line with what we've been planning."

Related content:

On Wednesday, the Pentagon released a list of more than three dozen units from nine states that will be supporting the border-security effort. The total number of troops, according to the release, will top 7,000, but "the number ... will change each day as military forces flow into the operating area."

Those units will join the roughly 2,200 National Guard members who've been operating along the border for months, which could bring the total up near 10,000 -- the low end of Trump's estimate. It's unclear whether the Pentagon is planning for the possibility of bringing that up to 15,000.

The decision to deploy thousands of active-duty forces to the border days ahead of the midterm elections has spurred debate in Washington, D.C., and beyond. Military leaders estimated last month that there were about 7,200 troops fighting the Islamic State group in Iraq and Syria.

On Wednesday, Defense Secretary Jim Mattis was asked if the move to use troops to harden vulnerable spots along the border is "a political stunt, as critics allege."

"The support that we provide to the secretary for Homeland Security is practical support based on the request from the commissioner of Customs and Border Police," he replied. "So we don't do stunts in this department. Thank you."

When pressed to address whether the troops are there to provide deterrence to the migrant groups, Mattis added: "We are there in support of the secretary of Homeland Security, who needs additional military assistance. We do this following storms; we do this in support of the Department of Homeland Security. This is a different aspect of it, but that's what we are doing."

About 1,000 active-duty troops had already arrived in Texas on Tuesday, where O'Shaughnessy said they would begin their mission of tightening up the border before moving on to other areas in Arizona and California.

Medium-lift helicopters will be used to transport border patrol agents and assess the border at night, he added, saying transport planes are also on standby, including C-130 Hercules and C-17 Globemasters that can move cargo and personnel.

-- Gina Harkins can be reached at gina.harkins@military.com. Follow her on Twitter at @ginaaharkins.