"Due to the fact that Democrats are unwilling to change our very dangerous immigration laws, we are indeed, as reported, giving strong considerations to placing Illegal Immigrants in Sanctuary Cities only," Trump tweeted. He added that, "The Radical Left always seems to have an Open Borders, Open Arms policy - so this should make them very happy!"
He later told reporters that he was "strongly looking at" the idea of releasing migrant families into those communities, though there were no immediate plans in place to implement Trump's threat.
"They're always saying they have open arms. Let's see if they have open arms," he said.
The reversal, which appeared to catch officials at the Department of Homeland Security off guard, came as critics were blasting news that the White House had at least twice considered a plan to release detained immigrants into so-called sanctuary cities, using migrants as pawns to go after political opponents.
Before Trump's comments, both the Department of Homeland Security and a White House official had insisted in nearly identical statements that that plan had been floated but then flatly rejected.
But not, apparently, by the president, who emphatically revived the idea.
"Sanctuary cities" are places where local authorities do not cooperate with Immigration and Customs Enforcement officials, denying information or resources that would help ICE round up for deportation people living in the country illegally.
They include New York City and San Francisco, home city of House Speaker Nancy Pelosi, who on Friday called the idea "unworthy of the presidency of the United States and disrespectful of the challenges that we face as a country, as a people, to address who we are - a nation of immigrants."
Chicago is also a sanctuary city, where immigrants are allowed to live and work freely without the fear of deportation, despite federal immigration laws.
"We want to make sure the population feels safe, when we talk about sanctuary cities that is what we are talking about, we are talking about an environment where people feel safe to go ask for services," said Laura Mendoza, Resurrection Project.
"Chicago is a welcoming city, I would say it's welcoming for anybody that is not named Donald Trump," said 36th Ward Alderman Gilbert Villegas.
Villegas is the chairman of the city's Latino Caucus. He called Trump's threats disgusting, and another example of the administration using the detainees at the border as political pawns.
Mayor Rahm Emanuel released a written statement that said, in part, "Small, medium and large cities across the nation are suddenly and rapidly identifying as sanctuary cities because of the abandonment of American values, ideals and cultural destiny under President Trump's watch."
Trump's statement did not shock the Resurrection Project, a community group in Pilsen. Mendoza said the administration should be looking at solutions rather than creating more problems.
"I think there is a crisis at the border that was created by the administration by having people mandatory detained, but murking [sic] the way that people ask for asylum," Mendoza said.
There were at least two versions of the sanctuary city plan that were considered, according to one of the people familiar with the effort. One would have moved people who had already been detained and were being held elsewhere to places with Democratic opponents of the president, while the other would have transported migrants apprehended at the border directly to San Francisco, New York City, Chicago and other spots.
Revelation of the "sanctuary cities" plan drew immediate condemnation from Pelosi and other Democrats.
Transporting large groups of immigrants to distant cities would be expensive and burdensome for Immigration and Customs Enforcement, which is already strapped for cash. The agency has said it doesn't have the resources for immigrants processed by the Border Patrol and Border Patrol in most southern border sectors and is now instead releasing families after a health screening and criminal background check, leaving local nonprofits to help them make travel arrangements.
Still, many "sanctuary" communities would likely welcome the immigrants in question and have nonprofit legal groups that could help them strengthen their legal cases to stay in the country.
The Department of Health and Human Services said this week that it had started scouting vacant properties that could be turned into facilities for holding migrant children in several cities, including Atlanta, Dallas, Houston, Phoenix, and San Antonio.
Those facilities would be licensed by each state and likely take several months to be approved and opened, separating them from the rapidly expanding emergency shelter at Homestead, Florida, and the now-closed tent facility at Tornillo, Texas.
ICE is tasked with arresting people living in the country illegally - including some who have been here for decades. Under the Trump administration, ICE has significantly stepped up arrests, including of people who have no U.S. criminal records.
In response, some cities have banished ICE from jails where agents could easily pick up immigration violators. Police in New York, Baltimore and Seattle rarely, if ever, disclose information about when suspected criminals in the U.S. illegally will be released from custody.
Democrats have said they will tackle immigration bills, possibly as soon as they return from their spring recess, and Senate Republican leader Mitch McConnell has indicated an interest in working on the issue.
WLS-TV contributed to this report