WASHINGTON — Republicans seized on an apparent gaffe by Joe Biden on Tuesday, after the US vice president remarked that America's middle class had "been buried in the last four years."

Biden, who is still President Barack Obama's running mate on the Democratic ticket, made the comments when speaking about the Republican camp's tax plans at a campaign event in Charlotte, North Carolina.

"How they can justify raising taxes on the middle class that's been buried in the last four years," he said, referencing the timeframe in which he and Obama have been leading the United States.

Biden's remarks triggered a quick response from Obama's Republican opponent in the November election Mitt Romney.

"Agree with @JoeBiden, the middle class has been buried the last 4 years, which is why we need a change in November #CantAfford4More," said a tweet from @MittRomney, the candidate's official Twitter account.

Romney's running mate Paul Ryan also issued a scathing response.

"Unemployment has been above 8 percent for 43 months. Our economy is limping along right now. Vice President Biden, just today, said that the middle class, over the last four years, has been 'buried.' We agree.

"That means we need to stop digging by electing Mitt Romney the next president of the United States," Ryan said.

A spokeswoman for the Obama-Biden campaign said the vice president had sought to explain that Americans had suffered in the past four years because of the decisions taken by previous Republican president George W. Bush.

"As the Vice President has been saying all year and again in his remarks today, the middle class was punished by the failed Bush policies that crashed our economy -- and a vote for Mitt Romney and Paul Ryan is a return to those failed policies," the spokeswoman said.

"With more than five million private-sector jobs created since 2010, the vice president and President Obama will continue to help the middle class recover and move the nation forward," she added.

However, the Republican barbs continued with former New Hampshire governor John Sununu, who served as White House chief of staff under President George H.W. Bush, chipping in.

"Today is a very important day in American history. Vice President Biden finally got something right," Sununu said in a campaign conference call with reporters.