“Detaining a child even for short periods under good conditions can have a serious impact on their health and development — consider the damage being done every day by allowing this alarming situation to continue,” Ms. Bachelet said.

Ms. Bachelet said that countries had a sovereign right to decide how to manage their borders, but that they still had to comply with their human rights obligations, and that the approach “should not be based on narrow policies aimed only at detecting, detaining and expeditiously deporting irregular migrants,” she said.

Several United Nations human rights bodies had determined that the Trump administration’s separation of children from their families likely constituted the sort of cruel and inhumane treatment prohibited in international law, she noted.

United Nations human rights officials have spoken out against the policy for more than a year since the administration started its zero tolerance policy of separating children from their parents. But officials in Ms. Bachelet’s office said she was prompted to add her voice by the dire conditions exposed in a report by the Department of Homeland Security’s independent watchdog.