The US Border Patrol on Sunday allowed reporters to briefly visit an old warehouse in South Texas, where it holds families arrested at the southern US border, while facing criticism over the Trump administration's "zero tolerance" policy and resulting separation of families.

More than 1,100 people were inside the large, dark facility that's divided into separate wings for unaccompanied children, adults on their own, and mothers and fathers with children.

The cages in each wing open out into common areas to use portable restrooms.

The overhead lighting in the warehouse stays on around the clock.

The Border Patrol said close to 200 people inside the facility were minors unaccompanied by a parent.

Another 500 were "family units," parents and children.

Many adults who crossed the border without legal permission could be charged with illegal entry and placed in jail, away from their children.

Reporters were not allowed by agents to interview any of the detainees or take photos.

Nearly 2,000 children have been taken from their parents since US Attorney General Jeff Sessions announced the policy, which directs Homeland Security officials to refer all cases of illegal entry into the United States for prosecution.

Church groups and human rights advocates have sharply criticised the policy, calling it inhumane.

Stories have spread of children being torn from their parents' arms, and parents not being able to find where their kids have gone.

A group of congressional lawmakers visited the same facility Sunday and were set to visit a longer-term shelter holding around 1,500 children - many of whom were separated from their parents.

"What we're doing as a nation is inflicting enormous harm on these kids and on the parents - what is totally unnecessary during this time when they're waiting for their asylum hearing," said Democratic Sen. Jeff Merkley of Oregon, who was denied entry earlier this month to children's shelter.

In Texas' Rio Grande Valley, the busiest corridor for people trying to enter the US, Border Patrol officials argue that they have to crack down on migrants and separate adults from children as a deterrent to others.

Agents running the holding facility - generally known as "Ursula" for the name of the street it's on - said everyone detained is given adequate food, access to showers and laundered clothes, and medical care.

People are supposed to move through the facility quickly. Under US law, children are required to be turned over within three days to shelters funded by the Department of Health and Human Services.

Padilla said agents in the Rio Grande Valley have allowed families with children under the age of 5 to stay together in most cases.

People gathered outside the facility on Sunday to protest against the Zero Tolerance Initiative.

"This is not the Texas I know." said Dora Saavedra from McAllen.

"I am so concerned for the future welfare of not only these little children, the teenagers, the parents, the trauma that they're experiencing, but also for the future of our mental state as a nation," she added.