After two children died in a month while in the care of the US border protection, medical checks will be carried out on every child in its custody.
An eight-year-old Guatemalan boy died on Christmas Eve while in the custody of US Customs and Border Protection (CBP).
Felipe Gomez Alonzo's death came as arguments over border security rumble on and a partial government shutdown is under way over President Donald Trump's request for border wall funding.
The boy had been in CBP custody on Christmas Eve when a US border patrol agent noticed he appeared unwell, and was transferred to a medical centre where he was diagnosed with a cold and fever, according border officials.
He was released a short time later with prescriptions for amoxicillin and ibuprofen, but later began suffering from nausea and vomiting and was readmitted to hospital.
Felipe died just before midnight, the agency said.
The body of seven-year-old Jakelin Caal, who died earlier this month, was returned this week to her village in Guatemala for her funeral.
In a statement, the CBP said it needs assistance from other government agencies to provide healthcare.
The agency "is considering options for surge medical assistance" from the coast guard and may request help from the US Department of Health and Human Services, the Department of Defence, and the Federal Emergency Management Agency.
Margaret Huang, executive director of Amnesty International USA, said the Trump administration's "policies of cruelty toward migrants and asylum-seekers at the border must cease immediately before any more children are harmed".
CBP Commissioner Kevin McAleenan said in a statement that the child's death was a "tragic loss."
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CBP processes thousands of children every month.
It has promised "an independent and thorough review of the circumstances," while the Guatemalan foreign ministry called for an investigation "in accordance with due process."