Congress has until midnight tomorrow to...
Mr. Kelly explained the administration is now looking at creating a wall across 800 miles (1,300 kms) of the 3,100 km stretch, a move that would include improving already existing fences.
"But overall, it's a deal we can probably support", he said.
The hardening stance reflects the influence of an emboldened Democratic base clamoring for a showdown with a president many on the left view as racist and untrustworthy.
Bipartisan Senate negotiators have come to terms on a deal to protect "dreamers", as young immigrants brought to the USA illegally as children are known.
Also, the chief of staff said the government was focussing on the situation of Dreamers - undocumented immigrants brought to the U.S. as children - whose protection from deportation lapses in March, unless Congress can reinstate it through a legislation that is the subject of intense debate and negotiations. If they have minor children, the minor children will come in.
Democrats' votes are needed to advance the stopgap measure through the Senate. While the tough talk carries weight in some Democratic circles, it's far from clear other voters will look kindly on using federal agencies as leverage in the fight over immigration.
Trump's comments came amid a flurry of tweets Thursday.
But Kelly has sought to manage expectations, and has bolstered Trump during some of his most controversial moments, including backing up his views on Confederate monuments and appearing in the White House briefing room to defend Trump's phone call with the grieving widow of a slain USA serviceman.
On Wednesday, Kelly darted between meetings on Capitol Hill meant to restart immigration talks after last Thursday's profanity-laden meeting.
Kelly made the remarks about Trump and the wall Wednesday at a closed-door meeting with members of the Congressional Hispanic Caucus, participants said, and he made similar comments later on Fox News Channel. Judy Chu (D-CA), nd House Minority Whip Steny H. Hoyer (D-MA).
Immigration activists have been gearing up for this fight for months. Trump has complained to associates that Kelly has clamped down on his free-wheeling style, including his veritable open-door policy at the Oval Office, suggesting it's a reason why he is spending more time in the residential area of the White House. "There are a lot of people trying to help out here".
Trump ended the legal shields on "Dreamers" previous year and gave Congress until March to renew them.
Kelly, while offering no timetable for when an agreement might be reached, gave an upbeat assessment of the state of play in the debate over legal status for illegal aliens who were brought to the United States as children. But fellow Republicans swooped in and reminded him that an immigration reform bill should also entail heightened border security, preferably in the form of a wall. That stiffened the resolve of liberal groups to push for a DACA deal this week, at the moment they feel Democrats have maximum leverage. "This week is an opportunity for Congress to reject that racism".
"Mexico has always done better with a Republican president and this will be no exception", he said.
"It's a smart fight", Stewart said.
Chief among them was the promise that the United States was going to build a wall along the entire Southern border and that Mexico would foot the bill. Immigration advocates are confident popular opinion is on their side - people from heads of companies like Amazon, Apple and Starbucks to TV personality Kim Kardashian have urged a deal.
The remaining half would be reallocated over time to immigrants who had been allowed to remain in the USA under temporary protected status.