"Thousands of children remain to be reunited. The world is watching the United States as some parents are deported to their home countries while their children are kept away. Some children are sent to the east coast while their parents are detained on the other side of the country," Vela said at the opening of the OSCE Parliamentary Assembly's annual session in Berlin.
Vela called US government's policy "appalling," stressing that as many as 3,000 children had been separated from their parents, with the country's authorities having no plan for their reunion.
READ MORE: Dozens of Migrant Children Remain Separated from Parents at US-Mexico Border
He recalled that a federal judge earlier in June had ruled to stop this "disgraceful policy" and expressed hope that the administration would fully comply with the order.
"What those children are experiencing is incomprehensible, unacceptable, and in blatant disregard to the values of the United States… Today for thousands of parents and children, the American dream has turned into an American nightmare," Filemon Vela noted.
The US President Donald Trump administration has come under intense criticism for its "zero-tolerance" immigration policy after recent images from a detention center in Texas showed children locked up in cages.
As a result of the policy, nearly 2,000 immigrant children were forcibly separated from their parents between April 19 and May 31 following arrests in connection with illegal entry into the United States, according to the Department of Homeland Security (DHS).
