WASHINGTON (Reuters) - The Pentagon is planning to send thousands more troops back to the U.S. border with Mexico, acting U.S. Defense Secretary Patrick Shanahan said on Tuesday, as Democrats in Congress questioned the merits of a mission that had declined in size.

FILE PHOTO: U.S. Army soldiers install a razor wire fence along Anzalduas International Bridge near the U.S.- Mexico border in McAllen, Texas, November 5, 2018. REUTERS/Delcia Lopez

The U.S. military says the new deployment will be largely on top of the roughly 2,350 active duty troops already involved in the border mission. Officials told Reuters the additional deployment could add 2,000 or more troops to the current total but declined to offer a final figure.

Still, U.S. officials, speaking on condition of anonymity, did not expect troop levels to return to their peak of about 5,900 active duty troops last November.

Shanahan said the upcoming increase in troops would help the Department of Homeland Security meet its goals of stringing up more concertina wire and expanding surveillance at the border.

“DHS has asked us to support them ... and we’ve responded with: ‘Here’s how many people it would take’,” Shanahan told a news conference.

The Pentagon says the U.S. military will operate mobile surveillance cameras in Arizona, California, New Mexico and Texas, a mission that is scheduled to run through Sept. 30. Some of the additional troops will also string up 150 more miles of concertina wire by March 31.

DEMOCRATIC SCRUTINY

The Pentagon first approved the high-profile deployment of active-duty U.S. troops to the Mexico border in October, ahead of U.S. midterm congressional elections. It was embraced by Trump’s supporters, including Republicans in Congress.

However, critics assailed the deployment as a political stunt to drive Republican voters to the polls and scoffed at Trump’s comparisons of caravans of Central American migrants, including women and children, to an “invasion.”

Democratic Representative Adam Smith, the new chairman of the House Armed Services Committee, held a hearing on Tuesday with Pentagon officials to shed light on the costs, scope and goals of the deployment.

He dismissed Trump’s assertions of a crisis at the border.

“There really isn’t much evidence of that crisis,” Smith said at the hearing, noting that the number of migrants apprehended in the United States after illegally crossing the border has fallen sharply in recent years.

Smith is skeptical about sending U.S. troops to deal with a crisis he believes would be better handled by judges. He told Reuters his committee may consider future funding restrictions for such missions.

“It’s undeniable that we have a significant increase in asylum seekers,” Smith told Reuters in an interview.

“But that’s not so much a job for the military as it is: we need more judges. We need to process them (the asylum seekers) more quickly,” he said.

He also noted that previous presidents had sought to mainly deploy National Guard forces to the border, as opposed to active-duty troops.Vice Admiral Michael Gilday, director of operations at the U.S. military’s Joint Staff, told the hearing active-duty troops were chosen over National Guard largely because they could deploy the fastest. He said the active-duty mission cost $132 million to date.

A separate, less controversial border mission involving the National Guard that began in April 2018 was expected to cost about $550 million by the end of September 2019.

Shanahan acknowledged Smith’s concerns.

“I’ve had lots of conversations with Chairman Smith ... He wants to ensure that there’s transparency and oversight in his role,” Shanahan said.