Alexandria Ocasio-Cortez was moved to tears as the migrant mother narrated her ordeal.

A US Congresswoman was moved to tears on Wednesday as a migrant mother narrated the story of her young daughter's death after being held by US immigration authorities last year.

Yazmin Juarez, an asylum seeker from Guatemala, spoke out at a congressional hearing staged amid a series of scandals over poor conditions suffered by detained migrants that has rocked Washington. She said 19-month-old daughter had contracted a deadly lung infection during a 20-day detention near the US border with Mexico.

The migrant mother's story left some members of a US House panel, including New York Democrat Alexandria Ocasio-Cortez, visibly shaken. Ms Ocasio-Cortez and several others hugged the woman after she spoke.

Alexandria Ocasio-Cortez, 29, is the youngest woman to serve in the US Congress.

Guatemalan woman Yazmin Juarez's daughter died after being held by US authorities last year.

Yazmin Juarez said she fled to the United States last year with her 19-month-old Mariee "because I feared for our lives in Guatemala."

She crossed the border and claimed asylum but says she and Mariee were "locked in a freezing cold cage for a few days," then moved to an ICE detention center, when her daughter became ill.

"I begged the doctors and medical staff to give her the care I knew she needed but they didn't," the mother said. "When ICE finally released us, I took Mariee... to a doctor and then to the emergency room. But it was too late. Mariee never left the hospital."

"The world should know," she said. "It can't be so hard for a country like the United States to protect kids who are locked up." The mother testified at the House Committee on Oversight and Reform that was examining treatment of refugees in US.

On Monday, the chief of the UN human rights body said she was "deeply shocked" by conditions under which migrants and refugees are held at US detention centers.

The Department of Homeland Security's watchdog released a report last week that warned of "dangerous overcrowding" in multiple centers holding thousands of migrants seeking to remain in the United States.

Most are fleeing violence and poverty in Central America.

(With inputs from AFP and Reuters)