The administration has characterized its policy as being about illegal immigration, though many of the detained migrants — including those in families that are split apart — enter at official border crossings and request asylum, which is not an illegal entry. It has also said that some adults falsely claim to be the parents of accompanying children, a genuine problem, and that it has to sort out their claims.

On Twitter, President Trump has appeared to agree that breaking up families was wrong, but blamed Democrats for the approach, saying that their “bad legislation” had caused it. In fact, no law requires separating children from families, and the practice was put in place by his administration just months ago.

The Times found in April that over six months, about 700 children had been taken from people claiming to be their parents.

The American Civil Liberties Union says that since then, the pace of separations has accelerated sharply. Lee Gelernt, deputy director of the group’s immigrant rights project, said that in the past five weeks, close to 1,000 children may have been taken from their families.

Last year, as Homeland Security secretary, John F. Kelly raised the idea of separating children from their families when they entered the country as a way to deter movement across the Mexican border.

Homeland Security officials have since denied that they separate families, but have also faced sharp criticism from Mr. Trump for failing to do more to curb the numbers of migrants crossing the border.

For the United Nations, it was a matter of great concern that in the United States “migration control appears to have been prioritized over the effective care and protection of migrant children,” Ms. Shamdasani said.