US President Donald Trump has defended his administration’s policy of separating migrant families in the face of mounting anger from within his own party as photos emerge of children locked in cages.

Mr Trump responded to a wave of outrage from within his own party, as well as the Democrats, over the forced separation of migrant children and parents at the US-Mexico border.

Speaking on Tuesday morning (AEST), the president blamed Democrats blocking his immigration policy for the separations and declared he would keep the US from becoming “a migrant camp”.

Former first lady Laura Bush called the separation policy “cruel” and “immoral”, while Republican Senator Susan Collins expressed her concern a day after Melania Trump appeared to question her husband’s policy.

Reporters and Democratic politician were on Monday allowed inside the Texas detention centre at the centre of the controversy.

While authorities did not allow photos or videos to be taken inside the converted warehouse, US Customs and Border Protection later released several images showing hundreds of unaccompanied children in cage-like detention cells.

“One cage had 20 children inside. Scattered about are bottles of water, bags of chips and large foil sheets intended to serve as blankets,” the Associated Press reported.

One teenager was quoted as telling an advocate who visited that she was helping care for a young child she didn’t know because the child’s aunt was somewhere else in the facility.

The teenager said she had to show others in her cell how to change the girl’s nappy.

Mr Trump, meanwhile, repeated his claims that the Democrats are to blame for the separations.

“I say it’s very strongly the Democrats’ fault,” he said at the White House.

“The United States will not be a migrant camp and it will not be a refugee holding facility. Not on my watch.”

Homeland Security Secretary Kirstjen Nielsen rejected accusations that her department was responsible for inhuman and immoral actions.

“We are doing none of those things. We are enforcing the laws passed by Congress,” Ms Neilsen said in an appearance before the National Sheriffs’ Association in New Orleans. 

Nearly 2000 children were separated from their families over a six-week period in April and May after Attorney General Jeff Sessions announced a new “zero-tolerance” policy that refers all cases of illegal entry for criminal prosecution.

US policy prohibits detaining children with their parents because the children are not charged with a crime and the parents are.

Ms Nielsen said that releasing parents with their children amounts to a “get out of jail free card” policy for those in the country illegally.

BREAKING: Border Patrol @CBP just gave us this video of the detention facility we toured yesterday in McAllen, Texas. We weren't allowed to bring in cameras, or interview anyone. To be clear: this is government handout video. pic.twitter.com/Zjy80qIZFZ

— David Begnaud (@DavidBegnaud) June 18, 2018

Speaking at the same conference, Mr Sessions called on Congress to pass Mr Trump’s immigration legislation if it wanted to end the separations.

“We do not want to separate parents from their children,” he said.

“If we build the wall, if we pass legislation to end the lawlessness, we won’t face these terrible choices.”

Mr Trump asserted that children “are being used by some of the worst criminals on earth” as a way to enter the United States.

“Has anyone been looking at the Crime taking place south of the border,” he tweeted.

Mrs Bush, wife of former president George W Bush, made some of the strongest comments from a Republican in an opinion piece for the Washington Post published on Monday.

“I live in a border state. I appreciate the need to enforce and protect our international boundaries, but this zero-tolerance policy is cruel. It is immoral. And it breaks my heart,” she wrote.

Mrs Bush compared it to the internment of Japanese-Americans during World War II, which she called “one of the most shameful episodes in US history”.

Mrs Trump released a statement via her spokeswoman Monday saying that she believed “we need to be a country that follows all laws,” but also one “that governs with heart”.

“Mrs. Trump hates to see children separated from their families and hopes both sides of the aisle can finally come together to achieve successful immigration reform,” spokeswoman Stephanie Grisham said.

Former Trump adviser Anthony Scaramucci told CNN that it “doesn’t feel right” for the Trump administration to blame Democrats and use the issue as a way of pressuring Democrats into negotiating on a Republican immigration bill.

House Democratic leader Nancy Pelosi said the policy “leaves a dark stain on our nation.”

“Ripping vulnerable little children away from their parents is an utter atrocity that debases America’s values and our legacy as a beacon of hope, opportunity and freedom,” she said on Tuesday.

-with agencies