The Trump administration has for the first time begun transferring suspected illegal immigrants to federal prisons, following additional demand for beds.
The transfer of 1,600 detainees underscores the surge in detentions under the President Donald Trump's "zero tolerance" immigration policy.
US Immigration and Customs Enforcement (ICE) said it was a temporary measure until other facilities could be found.
Some 1,000 of the detainees will go to a single prison in California.
Union leaders at prisons in California, Texas, and Washington state told the Reuters news agency they had little time to prepare for the large intake of detainees, and raised concerns about staffing and safety.
ICE is enacting a policy that requires federal prosecutors to criminally charge everybody caught illegally crossing the border.
Under previous administrations, first-time border crossers were usually put through civil deportation proceedings. Detainees awaiting civil hearings are usually housed in ICE detention facilities or county jails.
The 1,600 detainees transferred to prisons are expected to be there for 120 days while ICE finds space in new detention facilities, a spokeswoman for the agency said.
"Our federal prisons are set up to detain the worst of the worst. They should not be used for immigration purposes," said Ali Noorani, executive director of the National Immigration Forum.
"Federal prisons are for hardened criminals. They are not physically set up for immigrant landscapers looking for a job or fleeing violence."
ICE has attracted heavy criticism amid a crackdown on both illegal border crossings and people seeking asylum in the US. The Trump administration was recently forced to defend its stance on migrant children after it emerged that hundreds had been removed from their families, reports BBC.
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