(CNN) Hundreds of migrant children separated from their families at the southern border are in New York City. And Mayor Bill de Blasio says nobody told him they were there.

De Blasio said Wednesday that he just learned that 239 migrant children are in the care of Cayuga Centers in Harlem, which contracts with the federal government to help unaccompanied minors. The children include a 9-month-old, he said.

"How is it possible that none of us knew there were 239 kids right here in our own city?" the Democratic mayor said.

"How is the federal government holding back that information from the people of this city?"

Groups of children being dropped off at Manhattan foster care agency, which Mayor Bill de Blasio says houses more than 200 migrant children. It's unclear whether the children in the photo, who cover their faces, are among those separated from their parents.

'Stop this right now'

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