A US Border Patrol agent watches as people who've been taken into custody related to cases of illegal entry into the United States. US Customs and Border Protection's Rio Grande Valley Sector via AP US Customs and Border Protection's Rio Grande Valley Sector via AP

THE UN’S RIGHTS chief has condemned the “unconscionable” separation of migrant children from their parents at the US border, as First Lady Melania Trump made a rare political plea to end the deeply controversial practice.

The “zero-tolerance” border security policy implemented by President Donald Trump’s administration has sparked tears among migrant families and outrage on both sides of the political aisle.

“The thought that any state would seek to deter parents by inflicting such abuse on children is unconscionable,” Zeid Ra’ad Al Hussein said as he opened a session of the UN Human Rights Council in Geneva.

He called for Washington to immediately end the practice of “forcible separation of these children”.

Democrats, meanwhile, stepped up their opposition to the policy, describing it as “evil”.

“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.

They toured a converted Walmart supermarket that is now housing about 1,500 immigrant children, after which Merkley said “hurting kids to get legislative leverage is unacceptable. It is evil.”

Bracing for more child arrivals, the government plans to build camps at military bases in Texas.

Authorities 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.

Trump has said he wants the separations to end, but continues to blame opposition Democrats for the crisis, which critics say is of his own making.

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

Republican criticism

People who've been taken into custody related to cases of illegal entry into the United States, stand in line at a facility in McAllen, Texas. US Customs and Border Protection's Rio Grande Valley Sector via AP US Customs and Border Protection's Rio Grande Valley Sector via AP

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

The administration has decided “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 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.

In one incident that highlighted the crisis, five immigrants died and several others were injured after a high-speed chase with Border Patrol agents in Texas ended in a crash, US media said.

- © AFP 2018.