The US is bracing for a 20-year high in numbers of migrants arriving at the southern border, including thousands of children who are being kept in government-run detention facilities that critics say are inhumane.

A pandemic health order means that most adults and families are being summarily turned away, but the Biden administration has allowed unaccompanied children under the age of 18 to enter the US while their claims are processed.

The record breaking influx of children being held in these camps has led US officials to send in the Federal Emergency Management Agency - a government organisation that normally deals with major emergencies like natural disasters.

President Joe Biden has urged migrants not to attempt to travel to the US border. "Don't leave your town or city or community," he told them in an interview with ABC News.

How many children are being detained?

As of 14 March, US Customs and Border Patrol (CBP) agents were housing 4,200 children in detention centres.

The uptick is a 31% jump from less than a week earlier, when 3,200 migrant children - mostly from Central America - were reported to be held in US custody. The number of children kept over the three-day limit more than doubled in that time.

Almost 3,000 children have been kept for over 72 hours, the legal limit after which they are meant to be transferred to the custody of health officials in the Office of Refugee Resettlement (ORR).

Department of Homeland Secretary Alejandro Mayorkas has said these camps, which are often compared to jails or warehouses, are "no place for a child".

Pandemic restrictions and abnormally cold weather in Texas caused a delay in processing, Mr Mayorkas said earlier this month.

ORR facilities are generally better equipped to take children. The shelters feature play areas, classrooms and counselling services. The organisation is also tasked with finding families or homes where the children will remain until their immigration claim is heard by the courts.

According to CBS News, the BBC's partner in the US, 565 unaccompanied children are now entering US custody every day.

In February, around 9,500 children who were not accompanied by their legal guardian were detained by American officials.

Over 100,000 people in total were stopped from trying to cross into the US that month.

What are conditions like?

In short, we don't know.

Journalists have not yet been permitted inside the camps since President Joe Biden took office in January, although the White House says they will be.

But lawyers who represent children in the facilities say that they are being held in cramped and overcrowded conditions.

At a government-run tent city in Donna, Texas, holding 1,000 people, lawyers say children are being kept together so closely that they were able to reach out and touch their neighbour.

The children also said they have not been given adequate access to soap or food.

While in office, President Donald Trump faced outrage over the conditions inside border facilities holding children.

Images from inside those detention centres showed children overcrowded in metal cages, others sleeping under foil blankets.

Some of these Trump-era facilities - now renovated and upgraded - are being used again.

Despite concerns about coronavirus, health officials from the Centers for Disease Control and Prevention (CDC) have said these facilities can open at 100% capacity - though not all have yet.

What is the US doing to handle the surge?

Officials are scrambling to find more government buildings for the children.

A military base in Virginia and a California airbase run by Nasa - the US space agency - are being considered as other possible locations.

The Dallas convention centre will soon begin housing some 3,000 youths according to an email from city officials.

The facility, among the largest convention centres in the US, is located downtown and normally hosts business conferences.

Another camp originally set up for Texas oil workers in Midland is being managed by the American Red Cross charity, which has already deployed 60 volunteers and plans to send more in the coming days.

"Obviously we're going to have more kids," White House spokeswoman Jen Psaki said in a briefing. She added that this is due to the Biden administration "letting unaccompanied minors stay, and the last administration immorally kicked them out".

"So, of course, we have to look for facilities and places where we can safely and humanely have these unaccompanied minors in the interim."

What's behind the latest surge?

Lawmakers in Washington are pointing fingers over the current surge in migrants at the border.

"You can't help but notice that the administration changes, and there's a surge," said Republican Senator Bill Cassidy, echoing others in his party that have claimed that Mr Biden's promises of immigration reform have fuelled the increase in numbers.

President Biden dismissed this, in his interview with ABC. "The idea that Joe Biden said 'come'," he said. "I heard the other day that they're coming because I'm a nice guy. Here's the deal, they're not."

False rumours of an "open border" have been spreading in often violent and poverty-stricken communities where most of the migrants come from, experts say.

This is not the first surge of unaccompanied migrant children to trigger a crisis at the US-Mexico border. Similar humanitarian crises occurred in 2014 under President Barack Obama and in 2018 and 2019 under Mr Trump.

Experts say the reasons for migration are varied. Poverty, gang crime and natural disasters are some reasons that migrants may feel pushed to leave their home countries. Pandemic-related job losses could be adding to the surge this time, too.

Back-to-back hurricanes in Honduras last year may be one factor driving this year's surge, according to Mr Mayorkas.

In a statement on Tuesday, Mr Mayorkas said the US is on pace to see its highest level of southern border arrivals in 20 years, with the numbers steadily increasing since April 2020.

Policies in the US that favour migration, particularly those enacted by Democrats such as Mr Biden, could also be drawing migrants to attempt the dangerous journey.

How do Biden policies differ from Trump?

Under a pandemic-related health order called Title 42, the previous president's administration automatically deported anyone caught illegally crossing the border.

Mr Biden has only applied the order to adults and families, but not to unaccompanied children.

President Trump, who campaigned on a promise to crack down on immigration, had previously sought to deter immigration by arresting migrant families and separating the children to be housed with minors who had not been brought to the border by their parent or legal guardian.

The so-called "zero tolerance" policy drew widespread outrage and led Mr Trump to issue an executive order in June 2018 that halted the family separation policy.