Top administration officials have been discussing ways to increase pressure on countries with high numbers of citizens who overstay their visas in the U.S.
It's part of President Donald Trump's growing focus on immigration heading into his re-election campaign. The idea is just one on a long list under discussion by the administration as the influx of migrants at the border raises Trump's ire.
The ideas include Trump's threat to shut down the southern border and consideration of again separating children from parents. More subtle tweaks to the legal immigration system are being considered, too, including efforts to clamp down on visa overstays.
The administration has also been weighing targeting the remittance payments sent home by people living in the country illegally.
President Donald Trump now says that his proposal to send migrants to so-called sanctuary cities is taking effect.
Trump is tweeting that, "Those Illegal Immigrants who can no longer be legally held" in custody "will be, subject to Homeland Security, given to Sanctuary Cities and States!"
He also says that, "Congress must fix the laws and loopholes."
It's unclear, however, whether Homeland Security is taking any steps to implement the contentious plan. Lawyers there had previously told the White House that the idea was unfeasible and a misuse of funds. U.S. Immigration and Customs Enforcement is already strapped for cash.
The White House and Department of Homeland Security did not immediately respond to requests for comment on Monday.
Top House Democrats are asking White House and agency officials for internal documents about the administration proposal to send detained migrants to "sanctuary cities."
The chairmen of three House committees sent a letter Monday to the White House and Department of Homeland Security requesting emails and other documents discussing the idea, including its legal justifications and potential implications.
The letter says it's "shocking" the administration would consider such an action "for purely political reasons."
President Donald Trump has said he wants to explore the move as part of his drive to tighten immigration laws. The proposal was twice rejected by administration officials.
Officials in sanctuary cities limit cooperation with federal authorities seeking migrants in the U.S. illegally. They tend to be in Democratic areas.