US President Barack Obama believes Hillary Clinton is qualified to be president, the White House says, disagreeing with Bernie Sanders who thinks the Democratic frontrunner lacks what it takes to run the country.

“Yes,” White House spokesman Eric Schultz said on Thursday when asked whether Obama thinks the former secretary of state is qualified to be president of the United States.

“The president has said that Secretary Clinton comes to this race with more experience than any other non-vice president in recent campaign history,” Schultz said.

Democratic presidential candidate Sanders on Wednesday censured Clinton for her support of the invasion of Iraq in 2003, and said that she should apologize to the victims of the Iraq war, which she voted in favor of as a senator.

He also raised the issue of Clinton not being qualified to sit in the White House, citing “tens of millions of dollars” she is receiving from special interest groups through her super PAC.

“She has been saying lately that she thinks that I am, quote-un-quote, not qualified to be president,” Sanders said. “Let me just say in response to Secretary Clinton, I don’t believe that she is qualified if she is, through her super PAC, taking tens of millions of dollars in special interest funds.”

“I don’t think you are qualified if you have voted for the disastrous war in Iraq. I don’t think you are qualified if you supported the Panama free trade agreement,” he added.

On Thursday, Sanders reiterated that Clinton is not qualified to be president of America.

“This is campaign season, and campaigns are going to trade barbs back and forth,” said Schultz, the White House principal deputy press secretary.

Clinton running 'smear campaign'

The Sanders campaign on Thursday defended comments about Clinton not being qualified to be president, and accused her of conducting a "smear campaign."

Sanders’ campaign manager Jeff Weaver said the Vermont senator is prepared to strike back if she launches more unfair attacks against him.

“If they want to engage in that kind of a campaign, they’re going to see how a real New Yorker fights back,” Weaver told MSNBC.

“There’s been a lot of innuendo and unfair attacks, but they’ve really started to step them up, and as Sen. Sanders just said, he’s not going to take it,” he added.

“He’s the son of Brooklyn, born in New York. He knows how the rough and tumble of New York, and if that’s the kind of campaign they want to run, he’s fully prepared to run it,” Weaver continued.

Sanders, whose campaign was boosted by landslide victories in Alaska, Washington, Hawaii and Wisconsin, is hoping to cut further into Clinton’s lead in her home state of New York, where voters will cast their ballots on April 19.

Sanders has long been critical of US foreign policy and was an early opponent of the wars in Iraq and Afghanistan. Clinton tends to be more hawkish and inclined to use military force.

Sanders, 74, is also a leading proponent of issues such as income equality, universal healthcare, parental leave, climate change, and campaign finance reform in the US.

However, the incomes of Bill and Hillary Clinton put them at the upper end of the top 0.1 percent of earners in the US population, according to government data. The exorbitant pay for light work can distance them from the realities most Americans experience at their jobs.