A report from an internal US watchdog has found “dangerous overcrowding” in migrant detention centres in the south and urged authorities to act.

Jarring photos of facilities in the Rio Grande show 51 female migrants held in a cell made for 40 men, and 71 males held in a cell built for 41 women.

Adults were packed in standing room only cells for a week, with others held in overcrowded cells for over a month.

One facility manager called the situation “a ticking time bomb”.

“We are concerned that overcrowding and prolonged detention represent an immediate risk to the health and safety of [Department of Homeland Security] agents and officers, and to those detained,” inspectors said in the report.

The inspectors, from the US inspector general, visited seven sites throughout the Rio Grande valley in southern Texas.

At the facilities, the inspectors found that 30% of the detained children had been held for longer than the 72 hours permitted. Some had no access to showers or hot meals and had little access to clean clothes.

“When detainees observed us, they banged on the cell windows, shouted, pressed notes to the window with their time in custody, and gestured to evidence of their time in custody,” like facial hair, the report said.

They described detainees clogging toilets with blankets and socks in order to be released while the cells were fixed.

The report says these conditions directly contradict the US Customs and Border Protection’s (CBP) own standards.

The inspectors called upon the Department of Homeland Security (DHS) to take “immediate steps to alleviate dangerous overcrowding”.

According to the CBP, the Rio Grande has the highest volume of migrants on the southwest border, recording almost 250,000 apprehensions so far this year – marking a 124% increase from 2018.

On Tuesday, the DHS said they would build two tents to house additional migrants by the end of July. The agency added that fewer children are in their care than earlier this month.

The DHS had 2,800 children in detention on 7 June, according to government figures. By 25 June, less than 1,000 were in custody, it said.

In recent weeks, conditions at these facilities have been at the foreground of US politics.

Lawyers were given access by a judge to one facility in Clint, Texas, where they reported appalling conditions inside.

Children were “locked up in horrific cells where there’s an open toilet in the middle of the room” where they ate and slept, one of the lawyers told the BBC.

Last week lawmakers passed a bill to send $4.6bn (£3.6bn) to address the ongoing crisis at the border, amid growing outrage over the conditions.

The detention facilities “will shock the conscience of this country,” Democratic White House contender Beto O’Rourke said after a visit to a migrant centre.

Democrats, who have been touring the facilities in the past week, have decried the conditions inside, with one Democratic congresswoman claiming that border agents had told a detainee to drink from a toilet bowl.

Border officials have disputed her allegation, saying a sink in the cell that holds drinking water and drains into a toilet below was broken during her visit, and that migrants were given bottled water instead.

A Trump administration official, who did not wish to be named, told CBS News that “the toilets are connected to the sinks and the sinks dispense safe drinking water”.

“The sink on top of the toilets broke. But as soon as the sinks broke, border patrol put out jugs of water for migrants to drink right when that happened. The jugs were right there for everyone to drink.”

The row erupted as immigration officials said they would investigate a secret Facebook group of more than 9,000 current or former immigration agents. The group had allegedly been mocking migrants and using slurs and insults to describe visiting lawmakers.