Hundreds of migrant children are effectively being “incarcerated” in an immigration detention centre in Texas, according a journalist given access to the facility.

Nearly 1,500 boys are crammed into overcrowded rooms and reportedly given only two hours fresh air a day at Brownsville’s Casa Padre, the largest centre of its kind in the US.

Many were forcibly separated from their families under the Trump administration’s “zero-tolerance” policy after illegally entering the country, according to MSNBC correspondent Jacob Soboroff.

The number of unaccompanied migrant minors held across 100 centres has swelled to more than 11,000 in the weeks since the government launched the aggressive campaign to deter families crossing the border.

A 20-per-cent rise in children being held has brought the facilities to the brink of capacity, with the Department of Health and Human Services (HHS) now looking at building tent cities at military bases to hold thousands more minors.

Mr Soboroff, one of the first reporters allowed to tour Casa Padre after the zero-tolerance policy was introduced, said most children slept five to a room in dormitories designed only for four.

The government has allowed the centre, a converted supermarket run by private firm Southwest Key, to exceed that limit because of overcrowding.

“Kids here get only two hours a day to be outside in fresh air,” Mr Soboroff said. “One hour of structured time. One hour of free time. The rest of the day is spent inside a former Walmart.”

US congressman Luis Gutierrz speaks at Washington rally demanding end to family separations at US border

He added: “I have been inside a federal prison and county jails. This place is called a shelter but these kids are incarcerated.

“No cells and no cages, and they get to go to classes about American history and watch [Disney film] Moana, but they’re in custody.”

Children entering the facility for the first time are greeted by a mural of Donald Trump. Alongside images of the president, the American flag, and the White House, are words he once tweeted: “Sometimes losing a battle you find a new way to win the war”.

“[There are] presidential murals everywhere,” Mr Soboroff. “But that one is first.”

There is also a mural of Barack Obama.

“My fellow Americans, we are and always will be a nation of immigrants. We were strangers once, too,” reads his quote, in English and Spanish, taken from a 2014 speech in which he announced protections for undocumented immigrants.

Children line up for food in Casa Padre's canteen (The US Department of Health and Human Services ) (The US Department of Health and Human Services)

Lights at the shelter go off at 9pm and come on again at 6am.

Children eat and attend school in shifts. They are taught in numbered classrooms with tiny windows, through which they waved at reporters during the tour, also attended by the Washington Post and local media.

“You might want to smile,” journalists were told by a Southwest Key employee. “The kids feel a little like animals in a cage, being looked at.”

The centre’s detainees, all boys aged 10 to 17, stay an average of 52 days before they are placed in foster care or with other relatives.

“The lack of parental protection, and the hazardous journey they take, make unaccompanied alien children vulnerable to human trafficking, exploitation and abuse,” a spokesman for the Administration for Children and Families at the HSS told The Independent.

“In some cases many violent gangs, including MS-13, are exploiting loopholes in US laws to bring gang members into the United States or recruit unaccompanied alien children once placed with a sponsor,” he added.

The spectre of gangs has been used by Mr Trump's administration as part of its justification for the crackdown on undocumented migrants and the detention of their children.

However, staff at Casa Padre told MSNBC they had “never had an MS-13 member here, ever”.

Undocumented immigration across the US-Mexico border Show all 14 1 /14 Undocumented immigration across the US-Mexico border Undocumented immigration across the US-Mexico border Immigrant children, many of whom are separated form their parents, are housed in Texas' tent city Reuters Undocumented immigration across the US-Mexico border A two-year-old Honduran asylum seeker cries as her mother is searched and detained near the US-Mexico border Getty Undocumented immigration across the US-Mexico border Undocumented migrants ride on the top of a freight train referred to as the beast, or La Bestia Getty Undocumented immigration across the US-Mexico border A cage inside a US Customs and Border Protection detention facility in Texas Reuters Undocumented immigration across the US-Mexico border US Border Patrol Academy All new agents must complete a months-long training course at the New Mexico facility before assuming their posts at Border Patrol stations, mostly along the US-Mexico border Getty Undocumented immigration across the US-Mexico border US-Mexico border fence A group of young men walk along the Mexican side of the US-Mexico border fence in a remote area of the Sonoran Desert Getty Undocumented immigration across the US-Mexico border US-Mexico border fence in the US Man looks through US-Mexico border fence into the US in Tijuana, Mexico Getty Undocumented immigration across the US-Mexico border US-Mexico border fence US Border Patrol agent Sal De Leon stands near a section of the US-Mexico border fence while stopping on patrol on in La Joya, Texas Getty Undocumented immigration across the US-Mexico border US Border Patrol Academy US Border Patrol instructor yells at trainees after their initial arrival to the academy Getty Undocumented immigration across the US-Mexico border Memorial service in Guatemala Families attend a memorial service for two boys who were kidnapped and killed in San Juan Sacatepequez, Guatemala. Crime drives emigration from Guatemala to the United States, as families seek refuge from the danger Getty Undocumented immigration across the US-Mexico border Arrests on the border Undocumented immigrants comfort each other after being caught by Border Patrol agents near the US-Mexico border Getty Undocumented immigration across the US-Mexico border Detention holding facility A boy from Honduras watches a movie at a detention facility run by the US Border Patrol Getty Undocumented immigration across the US-Mexico border Mexican farm workers Mexican migrant workers harvest organic parsley at Grant Family Farms in Wellington, Colorado Getty Undocumented immigration across the US-Mexico border Mexican family in Arizona A Mexican immigrant family sits in the living room of their rented home in Tuscon, Arizona. The family that Arizona's new tough immigrant law had created a climate of fear in the immigrant community. Getty

The journalists’ visit to the shelter came days after a US senator was turned away and spoken to by police after trying to gain access.

The HSS refused to arrange a tour for Jeff Markley, who said it was “unacceptable that a member of Congress is not being admitted to see what is happening to children whose families are applying for asylum”.

Texas regulators have found hundreds of health violations, including a lack of supervision and medical care, at shelters for migrant children run by Southwest Key.

The policy of separating detained immigrant parents from their children has been driven by attorney general Jeff Sessions, who hopes it will dissuade families from crossing the border.