Child migrants and asylum seekers in the US have accused border agents of sexual assault and retaliating against those who complain about the conditions in which they are detained, according to a new report.

A 15-year-old Honduran girl told government investigators that one officer put his hands under her bra, pulled down her underwear and groped her during a security check, according to NBC News.

The man was “speaking in English to other officers and laughing” during the attack, she reportedly said.

It was one of a string of allegations against agents in Yuma, Arizona, that the US broadcaster said had been detailed in reports given by children to government case managers. The claims are just the latest in a series of investigations and testimonies bringing to light the inhumane treatment of some migrants under Donald Trump’s border regime.

One child wore soiled underwear for 10 days at the border station because he was scared to ask for a clean pair, NBC reported, while another boy said he was made to sleep outside despite wearing wet clothes after crossing a river.

'Dangerous' overcrowding of migrants in US border facilities Show all 6 1 /6 'Dangerous' overcrowding of migrants in US border facilities 'Dangerous' overcrowding of migrants in US border facilities Photos of detained migrants crowded into cells at the US border - with one holding up a sign reading simply "help" - have been released as part of a new report warning of "dangerous" overcrowding. The memo was sent to the Department of Homeland Security by its Inspector General, containing photos taken at border facilities in the Rio Grande Valley over a week in June DHS/OIG 'Dangerous' overcrowding of migrants in US border facilities Eighty-eight adult males held in a cell with a maximum capacity of 41, some signalling prolonged detention to OIG Staff on June 12 DHS/OIG 'Dangerous' overcrowding of migrants in US border facilities Overcrowding of families observed by OIG on June 11, at a border patrol facility in Weslaco, Texas DHS/OIG 'Dangerous' overcrowding of migrants in US border facilities Migrant families overcrowding a border patrol facility on June 11 in McAllen, Texas DHS/OIG 'Dangerous' overcrowding of migrants in US border facilities Fifty-one adult females held in a cell designated for male juveniles with a capacity for 40 at Border Patrol’s Fort Brown Station DHS/OIG 'Dangerous' overcrowding of migrants in US border facilities Migrant families overcrowding a border patrol facility on June 11 in McAllen, Texas DHS/OIG

A 16-year-old Guatemalan boy said that after he and his cellmates complained about the food they were given, US Customs and Border Protection (CBP) agents took away their sleeping mats and forced them to lie on the concrete floor.

“Our clients tell us that they have seen CBP agents kick other children awake, that children do not know whether it’s day or night because lights are left on all the time, and that they have had food thrown at them like they were wild animals,” advocacy lawyer Laura Belous told the broadcaster.

CBP told The Independent it had several ways people could file complaints, that the new allegations would be investigated and that the sexual assault claim was already being probed by the homeland security watchdog. The reported behaviour of its agents did not “align with common practice at our facilities”, a spokesman said.

It follows the publication of two investigations by the watchdog – the Department of Homeland Security’s office of the inspector general (OIG) – that revealed the squalid conditions in which migrants and asylum seekers were being held elsewhere.

Detention centres in Texas’ Rio Grande Valley were described by staff as a “ticking time bomb” after cells were seen crammed to bursting with people. “At one facility, some single adults were held in standing-room-only conditions for a week,” inspectors said, while children at three of the five sites inspected had no access to showers.

“Many” people were being fed only processed sausage sandwiches and “thousands” had been detained longer than the legal 72-hour limit – some for up to a month, during which time they had not been able to shower.

An earlier OIG report into conditions in California, Louisiana, Colorado and New Jersey detailed “egregious violations of detention standards” including people being served spoiled food, poor medical care and misuse of isolation cells.

And further testimony provided by a paediatrician revealed that babies and young children were being held in squalor at the US Border Patrol’s Ursula facility in McAllen, Texas, where children were left traumatised by their experience.

A mother whose baby had diarrhoea was denied fresh clothing while a 15-month-old with a fever had been detained for three weeks and was fed from the same unwashed bottle for several days, Dr Dolly Sevier said. The children she encountered were “totally fearful”, she added.

In its statement to The Independent on Wednesday, CBP said it treated “those in our custody with dignity and respect”.

Mr Trump has previously tweeted to say that people detained after migrating illegally or seeking asylum “are living far better now than where they came from, and in far safer conditions”. He added: “No matter how good things actually look, even if perfect, the Democrat visitors will act shocked & aghast at how terrible things are. Just Pols.”

Strengthening immigration rules, closing loopholes and instituting a merit-based system are the solutions to the crisis afflicting the border with Mexico, the president has said.

On Tuesday, homeland security officials reported a 28 per cent drop in the numbers of migrants encountered by CBP in June, from 144,278 in May to 104,344 last month. The tally included apprehensions between ports of entry as well as people who were deemed “inadmissible” by customs officers.

Congress has been forced to approve $4.5bn (£3.6bn) in emergency funds to ease dangerous overcrowding at border camps, after a huge rise in the number of children and families arriving from central America even in the face of Mr Trump’s hardline policies.

Homeland security leaders are expected to testify before the House Oversight Committee on Friday.