FILE - In this April 26, 2018, file photo, Defense Secretary Jim Mattis listens to a question during a hearing on Capitol Hill in Washington. The Trump administration’s request for the Pentagon to house migrants detained at the U.S. southern border and even provide lawyers to prosecute them is drawing congressional criticism that it puts undue strain on the military. (AP Photo/Jacquelyn Martin, File)

FILE - In this April 26, 2018, file photo, Defense Secretary Jim Mattis listens to a question during a hearing on Capitol Hill in Washington. The Trump administration’s request for the Pentagon to house migrants detained at the U.S. southern border and even provide lawyers to prosecute them is drawing congressional criticism that it puts undue strain on the military. (AP Photo/Jacquelyn Martin, File)

WASHINGTON (AP) — The Trump administration’s request for the Pentagon to house migrants detained at the U.S. southern border and even help prosecute them is prompting concern about strains to the military.

Some call it an inappropriate mission.

“We shouldn’t be militarizing border enforcement,” Rep. Ro Khanna, a California Democrat, said in an interview.

Defense Secretary Jim Mattis, who has made it his top priority to improve the military’s fitness for combat, argues that the Pentagon is nonetheless obliged to provide help with border enforcement.

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In recent days Mattis has accepted requests by the departments of Homeland Security and Health and Human Services to provide temporary housing on Air Force and Army bases for potentially tens of thousands of detained migrant families and unaccompanied children. This is in line with historical precedents for military assistance, Mattis argued.

“We have housed refugees, we have housed people thrown out of their homes by earthquakes and hurricanes; we do whatever is in the best interest of the country,” he recently told reporters. But he also has insisted that decisions about immigration policy and its security implications are not his to make. “I’m not going to chime in from the outside,” he said.

The Pentagon says it received a Department of Homeland Security request to house up to 12,000 detained migrant family members, starting with shelters for 2,000 people to be available within 45 days. The initial shelters are likely to be at Fort Bliss in Texas, but subsequent tent cities could be at two other bases in border states.

The Pentagon has indicated Mattis will accept these requests, but no steps have been taken yet to move migrants onto the bases.

Mattis has emphasized that members of the military will not be directly involved in detaining or securing migrants. By law, the military is barred from performing domestic law enforcement functions such as arresting people crossing the border, but it has sometimes provided support for border security, including monitoring surveillance imagery and repairing border fences.

Nonetheless, Mattis has been the target of pointed objections from senators critical of the Pentagon taking on a bigger role, including lending 21 military lawyers to the Justice Department to help prosecute immigration cases at the border.

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“Clearly, the military needs more, not fewer, lawyers available for its critical military justice practice,” Sens. Joni Ernst, R-Iowa, Kirsten Gillibrand, D-NY, and Patrick Leahy, D-Vt., wrote in a letter to Mattis last week. They pointed out the Pentagon already had said that military lawyers are too busy to expand the services of special victims counsel.

“Instead, we have now learned the services will be diverting these valuable resources to support a non-military mission,” the senators wrote, adding that they are “deeply troubled” by what they consider a misuse of military personnel.

The Pentagon has agreed to set up tents -- which it euphemistically calls “semi-separate, soft-sided camp facilities” -- at Fort Bliss, if needed, for detained migrant families starting this summer. It has also agreed to make housing available at Goodfellow Air Force Base in Texas for up to 20,000 unaccompanied minors detained for illegally crossing the southern border.

These shelter operations, which could last for months, are to be run by DHS or HHS or their contractors, not the Pentagon, but defense officials say it is possible that the Pentagon will end up erecting the shelters.

This is not unprecedented. The Pentagon temporarily housed several thousand unaccompanied detained minors in 2014 at military bases. This year’s problem has drawn wider attention and sharper criticism in part because of the administration’s now-suspended practice of separating children from parents who are detained for illegally crossing the border.

“We certainly should not be using our nation’s vital military bases for non-defense purposes,” Rep. Raja Krishnamoorthi, an Illinois Democrat, said earlier this month in arguing that Congress should block funding for migrant detention facilities on military bases.

Even some Republicans who support Trump’s crackdown on illegal immigration are unenthusiastic.

“I am not convinced at this point that housing them in our military installations is the best short-term answer, especially if it harms regular base operations, crowds our service members, and distracts from the mission of defense,” Rep. Jodey Arrington, a Texas Republican, said in a statement. His district includes Dyess Air Force Base, which is among bases under consideration for sheltering unaccompanied detained minors.

More broadly, Trump has urged the Pentagon to help pay for the border wall that he says is critically important to stemming the tide of illegal immigration. “Our Military is rich again,” he wrote on Twitter in late March, adding that the Pentagon should pay for the wall because it would keep “enemy combatants” out of the country.