U.S. President George Bush plans to use the National Guard to help keep illegal immigrants from entering through the Mexican border. He's expected to unveil the proposal Monday evening.

A White House official told The Associated Press that the National Guard would support the existing Border Patrol, which would maintain authority.

Stephen Hadley, the president's national security adviser, would not confirm the proposal but told CNN's Late Edition it was an option Bush was considering.

"It's not about militarization of the border," he said. "It's about assisting the civilian border patrol in doing their job, providing intelligence, providing support, logistics support and training and these sorts of things."

Bush's apparent plan could be a way to appease conservatives who want stronger border security, while he presses forward with another proposal to give foreigners temporary work permits for low-paying jobs.

The National Guard already has a small presence at the Mexican border. About 100 troops help with anti-drug smuggling operations, among other duties.

"I think what it would be is simply expanding the kind of thing that has already been done in the past in order to provide a bit of a stopgap as the Border Patrol build up their capacity to deal with this challenge," Hadley said on CNN.

On Sunday, Bush spoke with Mexican President Vicente Fox over the phone and assured him a military presence at the border would only play an administrative and logistical role, according to Fox's office.

Fellow Republican Senator Chuck Hagel of Nebraska is critical of Bush's plan to assign National Guard troops to patrol the border.

He told ABC's This Week the National Guard could do little to secure the border in the long run, since its numbers are already stretched thin in operations outside the country.

"We've got National Guard members on their second, third and fourth tours in Iraq," he said.

"We have stretched our military as thin as we have ever seen it in modern times. And what in the world are we talking about here, sending a National Guard that we may not have any capacity to send up to or down to protect borders? That's not their role."

Hagel argued a better solution would be a bill he helped introduce, and which the Senate is debating.

It would double the Border Patrol from 12,000 to 24,000 over the next five years.

"That's the way to fix it, not further stretching the National Guard," he said.

With files from The Associated Press