NEW YORK - United Nations Secretary-General Antonio Guterres warned on Monday that refugee and migrant "children must not be traumatized by being separated from their parents" as US President Donald Trump faces heavy pressure to stop such a policy at the US-Mexico border."Family unity must be preserved," UN spokesman Stephane Dujarric told reporters. "The Secretary-General believes that refugees and migrants should always be treated with respect and dignity, and in accordance with existing international law."Democrats and some in Trump's own Republican Party have strongly condemned the administration for separating nearly 2,000 children from their parents at the border between mid-April and the end of May. Medical professionals have said the practice could cause lasting trauma to children.