Overcrowding at Border Patrol stations in South Texas has become so acute in recent days that US authorities have taken the rare step of using aircraft to relocate migrants to other areas of the border simply to begin processing them, according to three Homeland Security officials.

The first flight left McAllen, Texas, on Friday, transferring detainees to Border Patrol facilities in Del Rio, Texas.

There are daily flights scheduled for the next several days, with two planned for Tuesday, according to officials who spoke on the condition of anonymity to describe the operations.

The flights are conducted by US Immigration and Customs Enforcement (ICE), but the detainees remain in the custody of Border Patrol, officials said.

Though ICE routinely uses aircraft to move detainees among its detention facilities, it is very unusual for Border Patrol to fly recent arrivals from one part of the border to another to perform routine booking procedures.

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Homeland Security officials requested the aircraft because Border Patrol urgently needs to move single adults out of the lower Rio Grande Valley of South Texas.

The agency is scrambling to make room for the large volume of families and children who have come across the border in dramatically higher numbers in the past several days, officials said.

One official said the US government has resorted to using aircraft because all available buses were already in use and authorities needed every available transportation option.

“This is the worst I have ever seen it, by far,” said one veteran Border Patrol agent in South Texas who was not authorised to speak to reporters.

The number of people taken into custody along the Mexico border has exceeded 5,500 each day for several days in a row, and Border Patrol currently has more than 17,500 people in holding cells and tent sites set up in parking lots outside stations, officials said.

That is a 30 percent increase from late March, when authorities said border agents and infrastructure had hit the “breaking point”.

Tents have been set up in the parking lots outside Border Patrol stations in the lower Rio Grande Valley cities of McAllen, Brownsville and Rio Grande City to ease overcrowding.

Emergency tents for families also have been erected in El Paso and at Camp Donna, a military site in the Rio Grande Valley.

To alleviate overcrowding in holding cells, Border Patrol in recent weeks has begun releasing migrants directly from its custody, instead of waiting for ICE to pick them up and either detain or release them.

But the sheer volume of people coming across the border in the past several days has swamped the agency’s ability to process families and children, so holding cells are filling with single adults because they are a lesser priority.

Border Patrol will use the flights to transfer some of those adults to Del Rio, where facilities are less overcrowded, instead of having to conduct releases, officials said. Each flight costs $16,000 (£12,300) and can transport about 135 adults.

Trump compares US-Mexico border to Disneyland and calls end to family separations a disaster

Homeland Security officials view the direct release of single adults as a red line, because they say that demographic group has the most potential to be deterred by enforcement efforts.

Carla Provost, the chief of Border Patrol, told lawmakers on Wednesday that authorities would “lose control” of the border if they had to begin releasing single adults, because that group is the only remaining demographic that can be detained and quickly deported.

“My greatest concern is that we will no longer be able to deliver consequences and we will lose control of the border,” Ms Provost told members of a Senate Homeland Security panel.

Adults who arrive with children typically are released from custody after a few days with an appointment to see an immigration judge because US courts have limited the amount of time minors can be held in immigration jails.

Department of Homeland Security (DHS) officials say this model – which President Trump decries as “catch and release” – is to blame for the border surge.

Authorities detained 109,144 migrants along the Mexico border last month, the highest total since 2007. More than 60% of those taken into custody were families or children.

Acting Homeland Security secretary Kevin McAleenan and acting Defence secretary Patrick Shanahan plan to travel to the Rio Grande Valley on Saturday to visit the McAllen border station to call for a “whole of government” approach to addressing the crisis, according to a DHS statement.