WASHINGTON (REUTERS, WASHINGTON POST) - A seven-year-old girl from Guatemala died of dehydration and shock hours after she was taken into United States Border Patrol (CBP) custody, the Washington Post reported on Thursday (Dec 13).

The girl and her father had been detained by immigration authorities on Dec 6 in New Mexico as part of a group of 163 people who approached US agents to turn themselves in, the Post said.

Early on Dec 7, the girl started having seizures, and emergency responders measured her body temperature at 41 deg C, the Post said. She was taken to a hospital, where she died, according to the Post.

"Our sincerest condolences go out to the family of the child," CBP spokesman Andrew Meehan said in a statement to the Post.

"Border Patrol agents took every possible step to save the child's life under the most trying of circumstances," Mr Meehan said. "As fathers and mothers, brothers and sisters, we empathise with the loss of any child."

US Customs and Border Protection did not immediately respond to a request from Reuters for comment.

A spokeswoman at Providence Hospital in El Paso, Texas, where the Post reported the child was taken, also did not immediately respond to a request for comment.

The names of the girl and her father were not released.

The agency, which typically provides food and water to migrants in its custody, is investigating the incident to ascertain whether appropriate policies were followed, the Post said.

The head of the Department of Homeland Security, which oversees Customs and Border Protection, will appear in front of the House of Representatives Judiciary Committee next week, Mr Jerry Nadler, the top Democrat on the panel, said on Twitter.

"We will be demanding immediate answers to this tragedy," Mr Nadler said.

US President Donald Trump has made toughening immigration policies a central tenet of his presidency and has vowed to build a wall along the southern border with Mexico.

This summer, his administration's "zero tolerance" immigration policy, which resulted in separating children arriving at the border with their parents, caused a national outcry. The policy was mostly reversed.

The child's death is likely to intensify scrutiny of detention conditions at Border Patrol stations and CBP facilities that are increasingly overwhelmed by large numbers of families seeking asylum in the US.

Though much of the political and media attention has focused in recent weeks on migrant caravans arriving at the Tijuana-San Diego border, large numbers of Central Americans continue to cross the border into Texas, Arizona and New Mexico.

The groups sometimes spend days in smugglers' stash houses or walk through remote areas with little food or water before reaching the border.

Arrests of migrants travelling as family groups have skyrocketed this year, and Homeland Security officials say court rulings that limit their ability to keep families in detention have produced a "catch and release" system that encourages migrants to bring children as a shield against detention and deportation.

In November, Border Patrol agents apprehended a record 25,172 "family unit members" on the south-west border - including 11,489 in the Rio Grande Valley Border Patrol sector in southern Texas and 6,434 in the El Paso sector, which covers far western Texas and New Mexico.

Migrants travelling as part of a family group accounted for 58 per cent of those taken into custody last month by the Border Patrol.