“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,” her spokeswoman Stephanie Grisham told CNN.

“She believes we need to be a country that follows all laws, but also a country that governs with heart.”

The president himself later tweeted: “The Democrats should get together with their Republican counterparts and work something out on Border Security & Safety. Don’t wait until after the election because you are going to lose!” he tweeted.

"They call it 'zero tolerance,' but a better name for it is zero humanity, and there's zero logic to this policy," said Senator Jeff Merkley of Oregon, after leading a group of Democratic lawmakers to the Mexican border.

"It's completely unacceptable under any moral code or under any religious tradition to injure children, inflict trauma on them in order to send some political message to adults somewhere overseas."

After touring a converted Walmart supermarket that is now housing about 1,500 immigrant children, Merkley said "hurting kids to get legislative leverage is unacceptable. It is evil."

‘It’s terrifying’: families crossing US border risk losing their children

The government has said that during one recent six-week period nearly 2,000 minors were separated from their parents or adult guardians – a figure that only stoked the firestorm.

Amid deep divisions, congressional Republicans have struggled to craft a viable immigration plan.

The Republican-led House of Representatives may vote in the coming days on two immigration measures – a hardline bill and a compromise measure that would limit legal immigration while also ending family separations.

Representative Sheila Jackson Lee accused Trump of lying by claiming he was simply following to the letter a previously existing law.

“The president is not telling the truth. There is no law, there is no policy that has allowed him to snatch children away from their families,” she said.

“I can assure you we’ll be fighting to the end to stop this ugly, vile programme that is harming children and creating massive child abuse.”

Earlier, Representative David Cicilline said the policy was “undermining the founding values of this country.”

Trump blasts Merkel to defend his policy of separating migrant families

“We saw the fear in the eyes of these children who are wondering when they will see their parent ever again. It’s a disgrace, it’s shameful and it’s un-American,” he added.

Immigration is one of the most divisive issues plaguing the Trump administration.

The number of separations has jumped since early May, when Attorney General Jeff Sessions announced that all migrants illegally crossing the US border with Mexico would be arrested, regardless of whether the adults were seeking asylum.

Since children cannot be sent to the facilities where their parents are held, they are separated, which the American Academy of Paediatrics has warned causes “irreparable harm” to the children.

Homeland Security Secretary Kirstjen Nielsen insisted, however, that “we do not have a policy of separating families at the border. Period.”

“For those seeking asylum at ports of entry, we have continued the policy from previous Administrations and will only separate if the child is in danger, there is no custodial relationship between ‘family’ members, or if the adult has broken a law,” she wrote on Twitter.

Chinese man arrested when he attends US immigration interview

Some of Trump’s fellow Republicans have said the policy must end.

“What the administration has decided to do is to separate children from their parents to try to send a message that if you cross the border with children, your children are going to be ripped away from you,” Senator Susan Collins told CBS television’s Face the Nation.

“That’s traumatising to the children who are innocent victims, and it is contrary to our values in this country.”

Laura Bush, the former first lady and wife of Republican ex-president George W. Bush, was unflinching in her rejection of the policy.

“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,” Bush, who lives in Texas, wrote in a Washington Post opinion piece.

Ex-president Bill Clinton, a Democrat, also weighed in.

“These children should not be a negotiating tool. And reuniting them with their families would reaffirm America’s belief in & support for all parents who love their children,” he tweeted.