Speaking as a partial government shutdown entered a ninth day due to an impasse over Trump's demands for funding of a wall at the US-Mexico border, Kelly told the Los Angeles Times: "To be honest, it's not a wall."
"The president still says 'wall.' Oftentimes frankly he'll say 'barrier' or 'fencing,' now he's tended toward steel slats," Kelly said.
"But we left a solid concrete wall early on in the administration, when we asked people what they needed and where they needed it," he added.
A former Marine general who led the military command responsible for Latin America, Kelly was Trump's Homeland Security secretary before becoming White House chief of staff in July last year.
His relationship with the president reportedly deteriorated however and he is to be replaced at the end of the year by Mick Mulvaney, the current budget director.
"Illegal immigrants, overwhelmingly, are not bad people," Kelly told the LA Times, adding that many had been manipulated by traffickers.
"I have nothing but compassion for them, the young kids."
The remarks were in sharp contrast to the rhetoric of the president who regularly appeals to his overwhelmingly white political base by taking a hard line on immigration.

'Pathetic immigration policies'

Trump has spoken of an "invasion" of migrants and complained of "many gang members and some very bad people" among a thousands-strong caravan of immigrants that traveled to the US in October.
Building a "wall" along the 2,000-mile (3,200-kilometer) US-Mexico frontier was a central plank of his 2016 election campaign, and he has tweeted about it almost 100 times this year alone.
Migrants from Central America say they want to reach the US to escape poverty and gang violence.
Trump has threatened to end aid to Guatemala, Honduras and El Salvador, despite a State Department announcement on December 18 that the US was ready to offer $4.5 billion in investment in Central America and southern Mexico, and that the administration was requesting an additional $180 million in assistance to the region.
On Saturday Trump blamed opposition Democrats "and their pathetic immigration policies" for the deaths of two Guatemalan children who crossed the border illegally with relatives who were taken into custody by US Border Patrol.
In further comments departing with the stance taken by his boss, Kelly said the way to halt illegal immigration was to "stop US demand for drugs, and expand economic opportunity" in Central America.
Kevin McAleenan, the commissioner of US Customs and Border Protection, said on ABC's "This Week" investment in Central America was one element of a "multi-faceted problem" that also requires funding border security.

'Wasting taxpayer dollars'

He called for a "sober-minded non-partisan look at our immigration laws" as part of the solution to an upsurge in family and child arrivals.
"We've asked for about 1,000 miles of wall... And what we're talking about is not just a dumb barrier," McAleenan said.
"We're talking about censors, cameras, lighting, access roads for our agents, a system that helps us secure that area of the border."
The opposition Democrats are refusing to provide $5 billion for Trump's border wall project and the president insists he will not fully fund the government unless he gets the money.
As long as the debate holds up approval of a wider spending bill, about 800,000 federal employees are not getting salaries and non-essential parts of the government are unable to function.
Kellyanne Conway, a close Trump advisor, said on "Fox News Sunday" that whether the border barrier was a "wall" or not was "a silly semantic argument," adding that the president had already compromised, since he had originally asked for $25 billion.
Democratic Congressman Hakeem Jeffries agreed on ABC's "This Week" that immigration reform and enhanced border security were needed.
"But we are not willing to pay $2.5 billion or $5 billion and wasting taxpayer dollars on a ransom note because Donald Trump decided that he was going to shut down the government and hold the American people hostage," he added.

1/1 A volunteer of the nonprofit Border Angels looks at a mattress used by people to land safely after jumping over the US/Mexico border fence, in Tecate, California