The US is sending 1,500 troops to bolster resources at its southern border ahead of an expected surge of migrants, a US official says.
They will join 2,500 National Guard members already in place and support the work of border agents.
Title 42, a Trump-era policy that gives the government power to automatically expel undocumented migrants, is set to expire on 11 May.
Officials expect a sharp increase in migrants to follow.
The additional military personnel, first reported by Fox News, will be deployed for 90 days to "supplement" the work of US Customs and Border Patrol (CBP) officers, the official told the BBC on Tuesday.
They will not be doing any law enforcement, the official said, but will instead assist with transportation, narcotics detection, data entry and warehouse support.
The BBC has contacted the Department of Defense and the Department of Homeland Security for comment.
The deployment is likely meant to ease the growing pressure on border agents as they prepare for a sharp increase in migrants at the southern border.
The number of migrants who crossed illegally into the US there during the 2021 fiscal year was the highest on record since at least 1960. Last year, that number topped 2.76 million - another record.
Troy Miller, the top CBP official, told Congress last month that his agency is preparing for more than 10,000 migrants to cross the border each day after the expiry of Title 42 - more than double the daily average in March.
Initially invoked in early 2020 at the onset of the Covid-19 pandemic, Title 42 allowed the US government to quickly expel migrants trying to cross its border with Mexico - including asylum seekers - with the stated intent of preventing the coronavirus from spreading in the US.
The Biden administration has unveiled a plan that would make it harder for migrants to claim asylum by requiring adult asylum seekers to use an app to book a meeting with US officials or first claim asylum in another country before reaching the US.
The app, known as CBP One, was rolled out in January and will remain in place regardless of Title 42.
Failure to comply would make migrants ineligible if they subsequently reach the border and would allow for those undocumented migrants to be swiftly deported.The plan - and its expedited removal of some migrants, including asylum seekers - prompted criticism from human rights groups.
Last month, the White House also announced it would set up brick-and-mortar immigration processing centres in Latin America, another effort to reduce the number of undocumented migrants crossing into the US.
The centres, with locations starting in Colombia and Guatemala, will screen migrants and determine whether they qualify for entry to the US.