Democrats have said that the situation is Trump's fault, and accused him of using the children as leverage to force them to negotiate on immigration and the president's long-promised border wall.

International organizations, human rights groups, and several countries have condemned the practice of family separations. Some Republicans have also joined Democrats in expressing concerns over it.

According to a White House official, US first lady Melania Trump "has been making her opinion known" to her husband that he needs to do whatever he can to keep families of migrants together.

Experts argue though Trump is seeking to galvanize his base with hardline law enforcement on immigration, moves like forcing family separations could backfire and hurt Republican candidates' chances in November's midterm elections.

"Separating mothers from small children is not a winning issue for him," Brookings Institution Senior Fellow Darrell West told Xinhua.

Trump announced his intent to sign the order at the White House earlier in the day, but reiterated his hardline stance on immigration and border security.

"We're keeping families together but we have to keep our borders strong," Trump said. "We still have to maintain toughness or our country will be overrun by people, by crime, by all of the things that we don't stand for and that we don't want."