Outgoing White House chief John Kelly said he had "nothing but compassion" for undocumented migrants crossing into the US and undercut the idea of a border wall in an interview Sunday that jarred with President Donald Trump's tough rhetoric on immigration.

Speaking as a partial government shutdown went into a ninth day due to an impasse over Trump's demands for funding for 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."  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.

But his relationship with the president reportedly deteriorated and he is due to be replaced at the end of the year by Mick Mulvaney, who currently serves as budget director.

Trump has been excoriated by opposition Democrats, who oppose his demand for USD 5 billion in border wall funding, after he blamed them for the deaths of two Guatemalan children in US custody in a tweet on Saturday.

"Illegal immigrants, overwhelmingly, are not bad people.... I have nothing but compassion for them, the young kids," Kelly told the LA Times adding that many had been manipulated by traffickers.