Overcrowded area holding families at a Border Patrol Centralized Processing Center in McAllen. (Reuters)

Government investigators warned of dangerous overcrowding at more migrant facilities on the southwest US border, publishing photos on Tuesday of packed cells in Texas' Rio Grande Valley where some children have no access to showers or hot meals.

A report issued by investigators for the Department of Homeland Security said supervisors raised concerns for the health and safety of detainees and agents, warning that the overcrowding represented a "ticking time bomb."

The DHS watchdog issued the report after visits to five US Customs and Border Protection agency facilities and two ports of entry in the Rio Grande Valley area during the week of June 10.

It came as President Donald Trump's administration pushed back against criticism of its migrant detention centers on the US-Mexico border.

Conditions at the detention centers have been a flashpoint since May when an internal DHS watchdog warned of similar conditions at facilities in the El Paso, Texas sector, west of the Rio Grande Valley, with migrants held for weeks instead of days, and adults kept in cells with standing room only.

Security incidents among men at multiple facilities included detainees clogging toilets in order to be released from cells, migrants refusing to return to cells, and special operations teams brought in to show that Border Patrol was prepared to use force, the report on Tuesday said.

An overcrowded fenced area holding families at a Border Patrol station is seen in McAllen. (Reuters)

Migrants banged on cell windows and shouted when investigators visited. One photo showed a man in a packed cell who was holding a message reading: "Help 40 Day(s) Here."

The Rio Grande Valley is the busiest area of the border for migrant arrests, which hit a 13-year monthly high in May during a surge in Central American families. At the time of the investigators' visits, U.S. Border Patrol was holding around 8,000 detainees in custody in the Rio Grande Valley sector, with 3,400 held longer than the 72 hours permitted.

DRINK OUT OF TOILETS

Members of a congressional group visiting facilities in El Paso on Monday said the migrants, many coming from Central America, were being kept in deplorable conditions and, according to US Representative Alexandria Ocasio-Cortez, told to drink out of toilets.

"I don't know what Alexandria Ocasio-Cortez is talking about," White House spokesman Hogan Gidley said in an interview with Fox Business Network, calling members of CBP "some of the bravest men and women on the planet."

"They put their lives in danger every single day. They provided three meals a day to people who are here illegally and unlawfully, two snacks in between," Gidley said.

The Democratic chairman of the House of Representatives Committee on Oversight and Reform said the panel had invited the acting heads of the Department of Homeland Security and CBP to testify on July 12 on the administration's border policies including the conditions at detention centers.

An overcrowded area holding families at a Border Patrol Centralized Processing Center is seen in McAllen. (Reuters)

"The Trump administration's actions at the southern border are grotesque and dehumanizing," Cummings said in a statement. "There seems to be open contempt for the rule of law and for basic human decency."

Trump has made a crackdown on illegal immigration a centerpiece of his domestic policy agenda and 2020 re-election bid. But his efforts to build a wall on the southern border have been blocked in Congress, and he was forced last year to backtrack after his "zero tolerance" border policy of separating migrant children from their parents provoked widespread outrage.

The detention facilities have become an issue among Democratic candidates vying to face Trump in the 2020 presidential election.

US Senator Cory Booker would "virtually eliminate immigration detention" if he wins the White House, his campaign said on Tuesday, including ending the use of for-profit detention facilities and minimizing the time unaccompanied children were in custody.

MIGRANT RIGHTS PROTESTS

Lisa Ayoub-Rodriguez, a Texas pediatrician who has been providing healthcare to migrant families, said she recently saw a mother suffering from dehydration with a baby whose fingers and toes were still blue after time in a detention center.

Several hundred people gathered in New York on Tuesday to demonstrate against the Trump administration's treatment of migrants, part of a planned nationwide day of protests by rights groups targeting members of the US Congress.

The demonstrations were fueled by fears that Trump's administration is preparing to move forward with a roundup in US cities of immigrants who are in the country illegally. Last month, Trump delayed the raids by two weeks.

An overcrowded area holding families at a Border Patrol station is seen in Weslaco. (Reuters)

Criticism of the CBP intensified after a report on Monday by the nonprofit news site ProPublica detailing offensive content posted on a private Facebook group for current and former CBP officers that included jokes about the deaths of migrants and sexually explicit references to Ocasio-Cortez.

Ocasio-Cortez, a first-term New York Democrat, suggested in a Twitter post on Tuesday that the CBP was a "rogue agency."

CBP condemned the Facebook group and acknowledged that the group may include a number of the agency's employees.

The migration flows from Central America have dropped sharply since hitting their highest level in more than a decade in May, as Mexico deployed thousands of militarized police as part of a June 7 deal with the United States to avoid US tariffs on Mexican goods and other enforcement measures.