The number of migrant children held without their parents by the US government has surged 21% since last month to 10,773 children, the Washington Post reported.

The uptick comes after the Trump administration imposed a new "zero tolerance" policy to prosecute migrants who cross the US border illegally.

The policy means that migrant parents who cross the border with their children are forcibly separated while they await criminal prosecution.

The Trump administration's new "zero tolerance" policy toward migrants who cross the US border illegally has driven up the number of migrant children held in government custody without their parents, the Washington Post reported Tuesday.

The US Health and Human Services Department said it was holding 10,773 migrant children in custody as of Tuesday — up 21% from the 8,886 it was holding a month earlier.

The surge comes in the wake of the Trump administration's new tactic to criminally prosecute every person who crosses into the US illegally, which requires them to be separated from any children they brought with them while they're detained.

But it's unclear exactly how many of the 10,773 children being held in government custody were actually forcibly separated from their parents — a Customs and Border Protection official told lawmakers at a hearing last week that 658 children had been separated from 638 adults between May 6 and May 19 under the new zero tolerance policy.

Many of the other children may have arrived at the border unaccompanied. They're typically held in government custody briefly before being placed with "sponsors," who are usually parents or immediate relatives of the children.

The shelters the children are staying in are at 95% capacity and are expected to add thousands of bed spaces in the coming weeks, one HHS official told the Post.

To house migrant children, HHS relies on "an existing network of approximately 100 shelters in 14 states."

HHS has also reportedly weighed housing migrant children on military bases, but the HHS official told the Post that measure is being considered only as a "last option."

The Trump administration has come under fire in recent weeks for its policies toward migrant children. The family separation policy sparked an uproar, particularly after the White House chief of staff John Kelly dismissed concerns that the policy was "cruel" during a recent interview with NPR.

"The children will be taken care of — put into foster care or whatever," Kelly said. "But the big point is they elected to come illegally into the United States and this is a technique that no one hopes will be used extensively or for very long."

Anger over the issue reached a boiling point last week, when a month-old piece of news resurfaced, prompting Trump critics to assail the government for losing track of 1,475 immigrant children who arrived at the border alone.

But both the Trump administration and immigration advocates have sought to tamp down concerns about those children, many of whom may have deliberately chosen not to tell the federal government where they are.