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.

“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.

If the programme does expire, Ms Nielsen said, she would not direct Immigration and Customs Enforcement to target immigrants who were previously covered and would generally not allow ICE to access personal information people submitted in DACA applications.