The US House of Representatives has approved a $4.5 billion aid package to help migrants arriving from Mexico, following an outcry over the squalid conditions in which they are being kept.
It comes as US government officials say they have returned about 100 children to a facility in Texas where lawyers said migrant children had been severely neglected and held without access to showers or enough food.
Clara Long, a researcher with Human Rights Watch who accompanied the lawyers to the migrant centre in Clint, said: "The three-year old before me had matted hair, a hacking cough, muddy pants, and eyes that fluttered closed with fatigue.
"His only caretaker for the last three weeks in a United States Border Patrol chain-link cage and then a cell... his 11-year old brother."
Ms Long added: "Children at Clint told us they don't have regular access to showers or clean clothes, with some saying they hadn't been allowed to bathe over periods of weeks and don't have regular access to soap".
US President Donald Trump, asked about conditions at the detention centres, said he was "very concerned".
He said "bad people" were using children to take advantage of lax US immigration laws. "It's a form of slavery what they're doing to young children," he said.
Meanwhile, the acting head of the US Customs and Border Protection agency John Sanders has resigned after just two months in the job, without giving a reason.
According to reports, Mr Sanders and other agency leaders had been overwhelmed by the scale of the crisis on the border with Mexico.
Arrivals of undocumented migrants at the southern US border have surged in recent months, with 144,000 taken into custody in May alone.