"It is unlikely that we will build a wall, a physical barrier, from sea to shining sea," Kelly told the Senate Homeland Security and Governmental Affairs Committee Wednesday morning.
Kelly did say it is still "very likely" that more border barriers will be built, but that they will be strategic.
"Physical barriers work," Kelly said.
Committee Chairman Ron Johnson and top Democrat on the committee Sen. Claire McCaskill both noted that the agents on the border say they would prefer fencing -- so that they can see what's happening on the other side of the border.
"Is the President OK with fencing instead of the wall?" McCaskill asked Kelly.
"The President has told me, 'Kelly, go do it. We need to protect the Southern border in any way that makes sense,'" Kelly said.
"The President knows that I'm looking at variations on the theme, and I have no doubt that when I go back to him and say, 'Wall makes sense here, high-tech fencing makes sense over here, technology makes sense over here,' I have no doubt he'll tell me to go do it," he added.
McCaskill praised Kelly's response -- and called it "embarrassing" that Trump won't admit his promise isn't doable. She added that members of Congress almost uniformly rejects the idea of a 2,000-mile concrete wall and won't appropriate funds for it and asked Kelly to "speak truth to power in that regard."
"As soon as we (can) stop this, 'from sea to shining sea we're going to build a wall and the Mexicans are going to pay for it' -- it's embarrassing," McCaskill said. "Everyone in Congress knows it's not going to happen. ... It appears the only person who won't say it out loud is the President."
In his written testimony submitted to the committee, Kelly said the money requested by the administration would go toward a variety of investments, including technology and fencing.
Border apprehensions drop
Also in the hearing, Kelly touted an "absolutely amazing" drop in the number of apprehensions at the Southern border on Wednesday as evidence that Trump's executive orders are already working.
As CNN first reported Tuesday
, the number of apprehensions at the Southwest border dropped by 35% in March from the month before, the second month in a row that apprehensions dropped substantially.
In both February and March, the drop in apprehensions defied a 17-year trend. Going back to 2000, according to Customs and Border Patrol data, apprehensions have increased every February and every March.
Kelly touted the numbers as an example of the effectiveness of Trump's early policies and executive orders on immigration enforcement already having an effect.
He noted that the drop in apprehensions includes a "dramatic reduction" in the number of families and minors making the trip -- a key focus of Kelly's as he emphasizes how dangerous and predatory the smuggling pathways that bring undocumented immigrants to the US are.