A 2015 court order now prevents the federal government from detaining children with their parents in detention centers for more than 20 days.
Obama's comments joined a firestorm of opposition to the Donald Trump administration's so-called "zero tolerance" policy begun in early May that caused the separation of more than 2,300 children from their parents after they crossed the US-Mexico border illegally.
The plan, as described by administration officials, would keep families together in federal custody while awaiting prosecution for illegal border crossings, potentially violating a 1997 court settlement limiting the duration of child detentions.
The move - if it retreats from his current stance that the administration is simply following the current law and needs congressional action to stop the separations - would mark a huge reversal by the President.
After long insisting that Congress alone could solve the problem, the Republican president seemed ready to take the matter into his own hands. The Alabama-based Southern Poverty Law Center said the order didn't go almost far enough.
Washington, Jun 20 US President Donald Trump's daughter and advisor Ivanka Trump urged her father to end family separations on the US-Mexico border, US media reported.
It is also expected to include a provision to prevent the government from separating young children from parents and guardians while in government custody. He said the Democrats are outraged over family separation, because they want to return to the harmful "catch and release" policy.
But there is no law requiring children to be separated from their parents at the border, and Trump, whose administration imposed the rule, could have stopped the practice at any time without an executive order.
Maryland Rep. Elijah Cummings, the top Democrat on the House Oversight and Government Reform Committee, pleaded with Republicans on the panel to end what he called "internment camps".
Polls show the public is overwhelmingly opposed to separating migrant children from their parents, though a majority of Republicans support the approach.
The House of Representatives planned to vote on Thursday on two bills created to halt the practice of separating families and to address other immigration issues.
Homeland Security Secretary Kirstjen Nielsen, the president and other officials have repeatedly said the only way to end the practice is for Congress to pass new legislation, though both Democrats and some Republicans have said the president could reverse it by himself. "Do we look away, or do we choose to see something of ourselves and our children?" he wrote.
"All of a sudden they weren't there anymore", Trump said.
In a conference call with reporters Wednesday evening about the order, Gene Hamilton, counselor to Sessions, referred questions about families that have already been separated to DHS and the Department of Health and Human Services.
He told House Republicans on Tuesday night he would support either of the immigration bills under consideration, but did not give a preference.
House Judiciary Committee Chairman Bob Goodlatte's bill, the Securing America's Future Act, provides a pathway to legal status for Dreamers while limiting legal immigration levels. "But at the same time I think you have to understand we are keeping families together, but we have to keep our borders strong".
Critics of the administration's zero-tolerance policy say that it's not only cruel to split families up, but also unjust to not have a mechanism in place to get them back together.
"A person's dignity does not depend on them being a citizen, a migrant, or a refugee".