President Donald Trump has again hit out in the escalating political crisis over the forced separation of migrant children and parents at the US-Mexico border.

Democrats have turned up the pressure over the policy, and some Republicans have joined the chorus of criticism.

Former first lady Laura Bush has called the policy “cruel” and “immoral” while GOP Sen Susan Collins expressed concern about it and a former adviser to Mr Trump questioned using the policy to pressure Democrats on immigration legislation.

President Trump continued to cast blame on Democrats, tweeting: “Why don’t the Democrats give us the votes to fix the world’s worst immigration laws? Where is the outcry for the killings and crime being caused by gangs and thugs, including MS-13, coming into our country illegally?”

Homeland Security Secretary Kirstjen Nielsen refused to apologise for enforcing immigration laws that result in the separation of children from their parents.

Speaking at a meeting of the National Sheriffs’ Association in New Orleans, she rejected criticism accusing DHS of inhuman and immoral actions.

“We are doing none of those things. We are enforcing the laws passed by Congress,” she said, calling on Congress to reform immigration laws.

Nearly 2,000 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 protocol 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.

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

He tweeted: “Has anyone been looking at the Crime taking place south of the border,” calling it “historic”.

The administration pushed back amid rising criticism from both sides of the political aisle.

In a guest column for the Washington Post on Sunday, Mrs Bush made some of the strongest comments yet from a Republican.

“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.

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

Underscoring the emotional tension, first lady Melania Trump, who has tended to stay out of contentious policy debates, waded into the issue.

Her spokeswoman said that Mrs Trump believes “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.