US authorities picked up almost 19,000 unaccompanied children at the south-west border in March, the highest monthly number on record.

The new figures from the US Customs and Border Protection agency put further pressure on President Joe Biden to humanely contain and process the growing number of child migrants as he rolls back many of Donald Trump’s hardline immigration policies.

CBP earlier this month said more than 172,000 people were encountered at the US-Mexico border in March. That includes almost 18,890 unaccompanied children, well above the previous high of 11,475 in May 2019.

The growing number of unaccompanied children crossing the southern border has left the US scrambling to find adequate holding facilities, leading to the reactivation of contentious temporary facilities run by CBP.

Pictures of the centres, which hold the children before transferring them to the custody of the US Department of Health and Human Services, have shown them sleeping on the floor in close quarters, despite the risks of Covid-19 transmission.

Republicans have blamed the Biden administration for encouraging migrants to make the dangerous journey north by undoing some of its predecessor’s hardline policies.

The figures on Wednesday further fuelled their criticism, with Ted Cruz, a Republican senator from Texas, calling the increase a “reminder that the crisis at the border is a direct result of Joe Biden’s failed policies”.

The White House has said it was working to fix a broken immigration system left behind by the Trump administration.

While the Biden administration has repeatedly emphasised that the US southern border remains closed, it suspended a Trump-era public health rule that allowed the immediate expulsion of unaccompanied minors. It has continued expelling other migrants, however.

Last month Biden put Kamala Harris, vice-president, in charge of US efforts to stop the flow of migrants from Central America over the southern border. Many of the people seeking to cross the border are fleeing poverty, violence and corruption in countries such as Guatemala, Honduras and El Salvador.

The US also sent three senior diplomats to Mexico last month to work on developing “an effective and humane plan of action to manage migration”, according to the White House’s National Security Council.

The administration has also dispatched the federal disaster agency to help manage the thousands of children and teenagers held in detention facilities and shelters.

Images shared on social media of a young child walking alone in the desert in Texas, sobbing and recounting how he had been travelling with a group but had been “dumped alone”, brought home the dramatic situation as the US confirmed the increase in apprehensions of minors.

“Can you help me?” sobbed the distraught child, wearing an anorak in a video. “I was coming with a group of people and they dumped me and I don’t know where they are,” he said in Spanish.

According to media reports, the images were recorded by a US border patrol officer on April 1.

In a telephone call with Harris on Wednesday, Mexican president Andrés Manuel López Obrador stressed the need to protect migrant children from human traffickers. “We are willing to join forces in the fight against human trafficking and to protect human rights, especially of children,” he tweeted.

However, he did not say how authorities planned to provide “appropriate” protection for unaccompanied minors.

López Obrador said on Thursday he and Harris would speak “periodically, to evaluate how we are doing” on the subject of migration.

Erika Guevara Rosas, Americas director at advocacy group Amnesty International, called the situation “very worrying”. She criticised both the US for holding minors in crowded facilities for longer than permitted, and Mexico for “illegal practices of forced return” of children and migrants to their home countries if at risk there.

She dismissed talk of human rights protection as “diplomatic speak”.

“There’s this disconnect between what they’re saying and what they’re doing. They use the language of protection and humanitarian policy but in practice, it’s the opposite — it’s militarisation and a totally disorderly migration that pushes people into a situation of risk and into the hands of organised crime,” she said.

Political polarisation in the US “doesn’t help at all”, she added, calling on the Biden administration to make significant changes to its foreign policy and end “really cruel” practices affecting migrants.

Biden’s envoy to the Northern Triangle of Honduras, Guatemala and El Salvador, Ricardo Zúñiga, is on a tour of the region. However, he left Honduras — where President Juan Orlando Hernández is facing allegations of complicity with organised crime — off his schedule, and El Salvador’s President Nayib Bukele reportedly stood him up.