Corey Lewandowski, Donald Trump’s former campaign manager, has drawn condemnation for mocking a young girl with Down Syndrome who was separated from her mother at the US-Mexico border.

Mr Lewandowski was described as a hateful “coward” by former Fox News host Megyn Kelly after he said, “Wah, wah” when the plight of the girl was brought up in a debate about the ”zero tolerance” policy that has seen thousands of children, some just toddlers, separated from their parents at the border.

The sound is commonly used as an effect to denote failure or comedic sadness.

Mexico’s foreign minister revealed the girl, aged 10, was being held in McAllen, Texas, while her mother was detained elsewhere. Luis Videgaray called the policy of housing children separately in detention centres “a clear violation of human rights”, and said he had raised it with the UN secretary-general.

The Trump administration has instituted a policy of charging every undocumented migrant or asylum seeker with a criminal offence, detaining adults separately from children. The youngest are kept in so-called “tender age” shelters in southern Texas.

During the debate on Fox News, Mr Lewandowski continued: ”When you cross the border illegally, when you commit a crime, you are taken away from your family because that’s how this country works.”

Mr Lewandowski was fired in June 2016 after police charged him with battery in a case brought by a female news reporter who said he forcibly grabbed her arm at a press event.

However, he appeared to have regained the president’s favour when, at a rally in Michigan earlier this year, Mr Trump called him up to the stage and praised him for not being a “pushover” or a “patsy”.

His sarcastic interjection was swiftly condemned, with Ms Kelly tweeting: “There is no low to which this coward Corey Lewandowski won’t sink. This man should not be afforded a national platform to spew his hate.”

Meghan McCain, the daughter of Arizona senator John McCain, said: “This is so horrible, even by Lewandowski standards.”

Undocumented immigration across the US-Mexico border Show all 14 1 /14 Undocumented immigration across the US-Mexico border Undocumented immigration across the US-Mexico border Immigrant children, many of whom are separated form their parents, are housed in Texas' tent city Reuters Undocumented immigration across the US-Mexico border A two-year-old Honduran asylum seeker cries as her mother is searched and detained near the US-Mexico border Getty Undocumented immigration across the US-Mexico border Undocumented migrants ride on the top of a freight train referred to as the beast, or La Bestia Getty Undocumented immigration across the US-Mexico border A cage inside a US Customs and Border Protection detention facility in Texas Reuters Undocumented immigration across the US-Mexico border US Border Patrol Academy All new agents must complete a months-long training course at the New Mexico facility before assuming their posts at Border Patrol stations, mostly along the US-Mexico border Getty Undocumented immigration across the US-Mexico border US-Mexico border fence A group of young men walk along the Mexican side of the US-Mexico border fence in a remote area of the Sonoran Desert Getty Undocumented immigration across the US-Mexico border US-Mexico border fence in the US Man looks through US-Mexico border fence into the US in Tijuana, Mexico Getty Undocumented immigration across the US-Mexico border US-Mexico border fence US Border Patrol agent Sal De Leon stands near a section of the US-Mexico border fence while stopping on patrol on in La Joya, Texas Getty Undocumented immigration across the US-Mexico border US Border Patrol Academy US Border Patrol instructor yells at trainees after their initial arrival to the academy Getty Undocumented immigration across the US-Mexico border Memorial service in Guatemala Families attend a memorial service for two boys who were kidnapped and killed in San Juan Sacatepequez, Guatemala. Crime drives emigration from Guatemala to the United States, as families seek refuge from the danger Getty Undocumented immigration across the US-Mexico border Arrests on the border Undocumented immigrants comfort each other after being caught by Border Patrol agents near the US-Mexico border Getty Undocumented immigration across the US-Mexico border Detention holding facility A boy from Honduras watches a movie at a detention facility run by the US Border Patrol Getty Undocumented immigration across the US-Mexico border Mexican farm workers Mexican migrant workers harvest organic parsley at Grant Family Farms in Wellington, Colorado Getty Undocumented immigration across the US-Mexico border Mexican family in Arizona A Mexican immigrant family sits in the living room of their rented home in Tuscon, Arizona. The family that Arizona's new tough immigrant law had created a climate of fear in the immigrant community. Getty

A dozen Republican senators have now written to Jeff Sessions, the attorney general, to says that separating families is not consistent with American values and ordinary human decency.

It came as a Guatemalan mother who had not seen her 7-year-old son since he was taken from her a month ago sued the Trump administration.

Beata Mariana de Jesus Mejia-Mejia has been released from custody while her asylum case is pending and believes her son, Darwin, may be in a shelter in Arizona.

“I only got to talk to him once and he sounded so sad. My son never used to sound like that, he was such a dynamic boy,” Ms Mejia-Mejia said, weeping. “I call and call and no one will tell me where he is.”