Donald Trump’s pick to oversee American immigration enforcement said a promised wall would not need to cover the entire US-Mexico border and signalled leniency toward young unauthorised immigrants.

The President selected Kirstjen Nielsen to head the Department of Homeland Security, which would place her in charge of the agency that manages immigration enforcement – one of Mr Trump's signature policies. He has vowed to build a wall spanning the border.

But Ms Nielsen said such a wall would not need to stretch from “sea to shining sea”, echoing DHS chiefs and others – including her former boss, White House Chief of Staff John Kelly – who say geography and cost would make that idea unfeasible.

Border fencing already marks some sections of the border, but rugged physical terrain and private property rights would make a comprehensive physical wall difficult.

Ms Nielsen was tabbed to fill a vacancy atop DHS left by Mr Kelly’s departure, part of an Oval Office shakeup that saw former chief Reince Priebus forced out.

The Trump administration has trumpeted the building of wall prototypes as a sign of progress on one of the President's central campaign promises. But a price tag that could run into the tens of billions, according to some estimates, makes for a hefty political hurdle. Congress so far has shown little inclination to allocate the needed funds, and Mexico is not offering to pay despite Mr Trump's assurances it would.

Mr Trump has also moved to allow the dissolution of an Obama-era executive program called Deferred Action for Childhood Arrivals (DACA) which allows immigrants who arrived in the country illegally when they were young to avoid deportation and receive work permits. Asked about DACA, Ms Nielsen said Congress “must” reach a deal to extend the programme and suggested she would take a permissive approach if Congress fails.

Life on the US-Mexican border Show all 12 1 /12 Life on the US-Mexican border Life on the US-Mexican border Tijuana, Mexico A house stands next to a section of the border fence separating Mexico and the US Reuters Life on the US-Mexican border Tijuana, Mexico The border fence in Tijuana stretches all the way to beach Reuters Life on the US-Mexican border Tijuana, Mexico Migrants can often be spotted trying to cross the rusty barrier Reuters Life on the US-Mexican border Tijuana, Mexico Joaquin, 36, a chef from Guatemala who says he was deported from the US Reuters Life on the US-Mexican border Tijuana, Mexico A shack stands next to a section of the border fence Reuters Life on the US-Mexican border Tijuana, Mexico Tourists pose for a picture at the border Reuters Life on the US-Mexican border Tijuana, Mexico A family burns rubbish near the fence Reuters Life on the US-Mexican border Tijuana, Mexico Joaquin makes a living by selling rubbish Reuters Life on the US-Mexican border Tijuana, Mexico ‘Neither Trump nor the wall is going to stop anyone,’ says Pedro Reuters Life on the US-Mexican border Tijuana, Mexico Mexicans have built homes right next to the fence Reuters Life on the US-Mexican border Tijuana, Mexico Mexican Carlos, 27, who says that he was deported from the United States, heats up tortillas at his house near the double fence that separates Mexico and the US Reuters Life on the US-Mexican border Tijuana, Mexico A girl climbs stairs near a section of the fence

“We owe it to them to find a permanent solution,” Ms Nielsen said in reference to the roughly 700,000 young immigrants who have secured DACA protection.