Former US president Joe Biden has returned to the national spotlight for the first time since leaving the White House, defending Social Security and accusing President Donald Trump's administration of gutting the critical programme.

Delivering his first public remarks since leaving office in January, Biden addressed a national conference of Advocates, Counselors and Representatives for the Disabled in Chicago on Tuesday, warning that Trump has "taken a hatchet" to the Social Security Administration (SSA).

"In fewer than 100 days, this new administration has done so much damage and destruction. It's kind of breathtaking," said the 82-year-old Democrat.

"They've taken a hatchet to the Social Security Administration, pushing an additional 7,000 employees out the door, including the most seasoned career officials. Now they're getting ready to push more out the door."

"Why are these guys taking aim at Social Security now?" he said. "They're following that whole line from tech startups — 'move fast, break things.' Well, they're certainly breaking things. They're shooting first and aiming later."

Presidential historian Timothy Naftali said it was important for Biden to speak out about Social Security, precisely because of his age.

"I understand why some Democrats would like Biden to fade away," Naftali said. "But a very important part of the Democratic base remains older people on Social Security, and Joe Biden has always been their champion."

'Sacred promise'

Biden echoed criticism from former Social Security Administration Commissioner Martin O'Malley, quoting him as saying: "They want to wreck it so they can rob it."

"Why do they want to rob it?" Biden continued. "To deliver huge tax cuts to billionaires and big corporations."

"It's a sacred promise we made as a nation. We know just how much Social Security matters to people's lives."

Biden warned that Republicans were setting the stage for deep cuts to Social Security and Medicaid, using the rising national debt as a pretext to strip funding from programs Americans have long paid into.

"Republicans — these guys — are willing to hurt the middle class and the working class in order to deliver significantly greater wealth to the already very wealthy," he said. "Who in the hell do they think they are?"

Biden dropped his reelection bid last July after a poor debate performance against Trump. Kamala Harris, his vice president, then lost the November election.

Biden left office in January with one of the lowest approval ratings for a post-World War Two president, at 38 percent, according to a Reuters/Ipsos opinion poll, and after a four-year term marked by high inflation and questions about his mental acuity.

Many Democrats blamed him for Trump's victory.