WASHINGTON - Increasing taxes on the wealthy would bring fairness to US taxpayers across the board, billionaire investor Warren Buffett said on Wednesday, backing the tax reform that President Barack Obama proposed in his State of the Union address.

On Tuesday, Obama proposed a minimum effective tax rate of 30 per cent on those who earn a million dollars or more, reviving his call for the so-called "Buffett rule," designed to prevent millionaires from paying a smaller share of income taxes than middle-class taxpayers.

Buffett published an editorial in August stating that his own 17.4 per cent effective tax rate was lower than that paid by wage earners in his office. He also said wealthy people should pay more taxes.

"The question is what is fair when you have to raise multi-trillions to fund the United States of America," Buffett said in a joint interview with his long-time secretary, Debbie Bosanek, on ABC News.

"(Raising taxes) will not change my behavior. I have paid all different kinds of rates and I've always been interested in making money. I believe this should be a defining issue. Debbie works just as hard as I do and she pays twice the rate I do,"said Buffett, a Democrat and Obama supporter.

Bosanek pays a tax rate of 35.8 per cent on her income, ABC said. She attended Tuesday's State of the Union address as a guest of first lady Michelle Obama.

"I just feel like an average citizen. I represent the average citizen who needs a voice," Bosanek told ABC. "Everybody in our office is paying a higher tax rate than Warren."

Buffett said he does not blame Republican presidential candidate Mitt Romney or other wealthy people for paying lower tax rates than most Americans and challenged Congress to make a change.

Buffett also lashed out at assertions by some Republicans that the "Buffett rule" is class warfare, the report said.

"If this is a war, my side has the nuclear bomb," Buffett said. "We have K Street (Washington lobbyists). We have Wall Street. Debbie doesn't have anybody. I want a government that is responsive to the people who got the short straw in life."

Shortly after the proposed millionaires' surtax was first raised, Senate Republican leader Mitch McConnell told NBC's"Meet the Press": "With regard to (Buffett's) tax rate, if he's feeling guilty about it, I think he should send in a check."