Democrats expressed concerns Wednesday about President Trump's executive order, which will allow parents who have crossed the U.S. border illegally to remain with their children. The move, some Democrats argued, simply detains illegal immigrants "indefinitely."

The order Trump signed allows children to stay in detention with their parents for an extended period of time. It does not, however, end the "zero-tolerance" policy that criminally prosecutes adults entering the country without proper documentation.

TRUMP SIGNS EXECUTIVE ORDER TO STOP FAMILY SEPARATIONS AT BORDER

Trump's order came after he and other officials repeatedly said only Congress had the power to stop children and their parents from being separated.

Sen. Bernie Sanders, I-Vt., tweeted Wednesday night that the executive order "merely replaces one inhumane act with another."

"In response to the overwhelming public outrage at his policy of tearing children away from their parents at the border, this administration thinks the appropriate response is to indefinitely detain families," Sanders said.

The senator said he's "hopeful ... the courts will step in to rein in these unlawful actions."

Democratic Sen. Dianne Feinstein, of California, shared similar sentiments, tweeting: "It’s extremely troubling that the president’s executive order would require immigrant families with children to be detained indefinitely."

"This Executive Order doesn’t fix the crisis," Sen. Kamala Harris, D-Calif., wrote. "Indefinitely detaining children with their families in camps is inhumane and will not make us safe."

Republican Sen. Marco Rubio, however, tweeted that to say immigrant children are detained in the U.S. "'indefinitely & indiscriminately' ... is just PATENTLY FALSE." He argued that using the term "indefinite detention" is "disingenuous."

The separations stem from the administration's "zero tolerance" immigration policy, which aims to prosecute all illegal border crossers. The policy moves adults to the custody of the U.S. Marshals Service and sends many children to facilities run by the Department of Health and Human Services.

The policy had led to a spike in family separations in recent weeks, with more than 2,300 minors separated from their families at the border from May 5 through June 9, according to the Department of Homeland Security

Trump said Wednesday that while "we're going to have strong, very strong borders ... we're going to keep families together."

Children, because of a 1997 order and related decisions, cannot be detained for longer than 20 days with the adults. A senior Justice Department official told The Associated Press that hasn't changed.

Fox News' Chad Pergram and The Associated Press contributed to this report.