FILE PHOTO: U.S. Defense Secretary Jim Mattis testifies before the House Armed Services Committee hearing on Capitol Hill in Washington, U.S., February 6, 2018. REUTERS/Joshua Roberts

WASHINGTON (Reuters) - U.S. Defense Secretary Jim Mattis said on Thursday that service members in the Deferred Action for Childhood Arrivals, or DACA, program will not be deported, other than a few exceptions, even as lawmakers debate the fate of children brought to the United States illegally.

Until now, the fate of about 800 service members in the program had not been clear.

“We would always stand by one of our people, and I have never found the Department of Homeland Security unwilling to take any call from anyone on my staff if we in fact found someone who had been treated unjustly,” he told reporters.

He added that the only exception was if the service member had committed a “serious” felony or a federal judge had signed deportation orders.

Mattis said the move applies to immigrants who had already signed up for the military or were waiting to go into boot camp, as well as veterans who had received an honorable discharge.

President Donald Trump campaigned in 2016 promising tougher rules for immigration. In September, he said he was ending the DACA program created by his Democratic predecessor Barack Obama, effective in March, unless Congress came up with a new law.

The program protects about 700,000 people, mostly young Hispanic adults, from deportation and provides work permits.

Lawmakers have struggled to reach a deal on an immigration bill, despite broad public support for helping Dreamers.

Trump has said any immigration deal must include billions of dollars to build a wall on the border with Mexico.